I overcame size with mechanics (Edwin Moses)
Almost the first thing that someone new to mirrorless cameras asks is this: how good are they?
Well, they're quite good, as long as you don't buy one in pink.
As with many things in life, you gain something and lose something when you pick a smaller or larger product. In the case of interchangeable lens cameras, that can get a little murkier than usual, as they're quite a range of product in this category. But a few things are clearly evident.
First, we have the obvious: you lose the mirrorbox when you step down from a DSLR to a mirrorless camera. With that lose comes the loss of an optical viewing system (it will be replaced with electronic screens, which are decidedly not the same thing as a clear optical view). In most mirrorless cameras, you also lose phase detect autofocus (the Nikon 1 is an exception). That last bit is important because phase detect autofocus generally has faster subject acquisition (at a minor expense of accuracy) and far better focus tracking capabilities than contrast based systems.
The loss of the mirrorbox also means that you lose a great deal of bulk. The mirror inside DSLR cameras pushes the lens mount forward from the sensor (the swinging mirror needs clearance). In a typical cropped sensor DSLR (APS, DX), that probably adds an inch to the depth of the camera, though there's a great deal of variation between models, so it's impossible to come up with an exact number. Still, a mirrorless camera, all else equal, will typically be thinner than a DSLR. It also may be less tall if it doesn't have an electronic viewfinder (EVF) to replace the optical one in the DSLR.
The mirrorless diet tends to produce cameras that are lighter and less bulky, all else equal. Some mirrorless cameras get very close to compact camera size, yet retain higher capabilities.
No mirror also means less vibration that might impact slow shutter speeds and promote a slight camera motion to pixels in some conditions.
But there are losses that might not be so great, too.
Most mirrorless cameras leave the sensor exposed (shutter is left open when not taking a picture, and there's no mirror in front to quasi-protect the sensor, either). Coupled with the fact it is so close to the mount, that makes it easy to get water, dirt, or something else on them, or to even damage them if you aren't mindful of their vulnerability.
You're not looking at reality (either via the rear LCD or a built-in EVF): you're looking at a time-delayed version of reality. It takes time to grab an image from the sensor, shuffle it through the camera's electronics, and display it on the electronic displays. On some cameras this "lag" is slight, while on others it's significant. In any case, you've lost seeing things in real time as you're composing. (Most people adjust to this with practice, even on the cameras with the most lag, but it makes it more difficult to capture peak action.)
With some cameras, mostly low end, we also lose some sophistication. The camera makers think these cameras are more likely to be bought by non-DSLR users, and thus while they often some DSLR-like control in them, this is usually watered down in control capability, flexibility, feature depth, or all of the above. High-end mirrorless cameras (the GH4, E-M1, X-T1 come to mind) are pretty much the equivalent of fairly high level DSLRs though.
The smaller size of most mirrorless cameras means that buttons and controls tend to get smaller, too. Using many of these cameras with gloves on (winter shooting) can be a bit of a problem. I've got one mirrorless camera that has maybe a dozen small buttons on it that are problematic to find and use with anything but the thinnest gloves on. And I mean thin gloves, as in glove liners.
It's not necessarily a loss, but if you're coming from bigger DSLRs that use CompactFlash cards, the smaller size of most mirrorless cameras means they use the smaller Secure Digital (SD) cards. A few use very small microSD cards, so take a close look at that.
One potentially big loss is battery oomph. The smaller sizes of most mirrorless cameras means that they often pack smaller batteries, as well. Given that the LCD is always on with these cameras, you'll get significantly shorter battery life than you do with even the low-end DSLRs (unless you always shoot with your DSLR in Live View).
Many of the mirrorless cameras have other losses, as well: watch out for weak flashes, missing remote controls, missing GPS support, missing hand grips, and a host of other options. While these are called "system cameras" by the makers, the system isn't always fully accessorized.
I've probably gotten you pretty depressed with all the losses I describe, so the gains better make up for it, right?
Autofocus on most of these mirrorless cameras is now competitive with low-end DSLRs, at least for initial subject acquitions. But the good news is that, because they use contrast-based autofocus, they tend to be better at things like face recognition and when they say they're in focus, they're in focus. Not ever so slightly off focus because the system determined the phase detect data was "within tolerance."
The reduced size means reduced carrying issues. In some cases, that can be considerably reduced from even the low-end DSLRs. Several of the mirrorless cameras are what I'd call jacket-pocketable, even with their kit zoom lens mounted. There's a big difference between two to five pounds of weight hanging from your neck all day and a pound of stuff in your pocket. A big difference that can get bigger when we get to lenses (more in a bit).
Likewise, the weight limits some airlines are now imposing on carry-ons come into play. On assignment with my DX DSLR gear I'm typicall in the 20-30 pound pack range. With FX gear, I'm almost always over 30 pounds. And that's for a minimal kit. My usual m4/3 kit not only weighs less than 10 pounds, but it fits in my laptop bag. Yes, in my laptop bag. That's two camera bodies, four lenses that take me from 18mm to 300mm equivalent, a bunch of accessories, and my laptop. (Some of you are asking "which laptop bag is that?" Get the Tenba Messanger Small bag. It'll hold a 15" laptop and all that mirrorless gear I just mentioned, and still have some room left over. Use the link, below to order one and support this site.)
With the smaller size tends to come smaller lenses. Not always, but with some models there can be significant savings in size in lenses, too. First, because the mirror box is missing, the lenses have to be designed for a short back focus. While that alone doesn't dictate small lenses, it has put a different twist on what consititutes the right sized lens. Different companies have gone at this differently.
Olympus, for example, has put a strong emphasis on small lenses. Coupled with their 2x crop sensor, which already has a small image circle, Olympus has thought to make many of their lenses “collapsing”—they're smaller when not being used than when they are. The 9-18mm and the 14-42mm kit lens are two good examples of lenses that are very small while traveling.
Sony is at the other extreme. It appears that they originally pretty much took their APS designs and did little tweaking to make them smaller. Other than the 16mm f/2.8 lens, none of Sony's E-Mount lenses can be considered small, though many of them are fairly light in weight. The FE-Mount for the Sony A7 cameras is more of the same, in spades.
So the size/weight/traveling benefits vary between the different mirrorless options. Right now you can pick between the ridiculously small Pentax Q and the approaching full frame sized (except for body size) Sony FE mount cameras. So the gain here is variable. Make sure you know what it is you're seeking in terms of light/small and make sure that the camera you choose attains that.
Another thing that people like to point out is that, since these cameras do very respectable (and in some cases, like the GH4, superb) video, you can stuff them in places that you normally can't get a regular video camera. Olympus Pen cameras were used to film in places on the track for the movie Secretariat, for example, including in the starting gate. Thin is good when you're trying to pin a camera to a wall.
It may not seem like it at first, but there are certainly other gains, some of which you might not have thought of.
For example, when I use my mirrorless cameras, I can often shoot without looking like I'm shooting. Yes, I know that I can do something similar with Live View on many DSLRs, but the small size coupled with "not-at-eye" shooting (especially with the tilt/swivel LCDs) means people just don't pay a lot of attention to me when I'm shooting with these cameras. It makes it easier for capturing true candids, I think (obviously some cameras are better at this than others, but all the mirrorless cameras look at lot less professional and "look at me while I'm shooting you" than my big FX DSLRs.
But What About Image Quality?
I've been conspicuously avoiding talking about image quality for a reason: there's quite a variation here, and it also depends upon what you're trying to accomplish.
At the far end of the scale, the sensor on the Pentax Q is so small it's...well, it's a sensor from a compact camera. That means that the Q tends to perform like a compact camera when it comes to image quality, which is to say that it struggles with very low light situations.
At the other extreme, we have the Sony offerings with their state-of-the-art full frame sensors. The A7r clocks in at 36mp, the A7 at 24mp. Both shoot as well as the best full frame DSLRs in low light, plus have pixel counts that let you print large.
But is that what you really need? Are you really shooting at ISO 3200 and above and are you really printing at 19" and above? Because if you aren't, some of the smaller sensor models in the mirrorless realm might work perfectly fine for you.
Moreover, the m4/3 models offer a lot of fast lens choice, and fast lenses are just as instrumental as sensor when tackling low light. Compare the 12mm f/2 on the Olympus Pens with the 16mm f/2.8 on the Sony E-Mount. From an exposure standpoint, you're shooting a stop faster with the same angle of view, so if the Sony is at ISO 1600, the Pen is at ISO 800 with the same shutter speed. (Yes, I know there are still some other factors involved, but let's not get down in the weeds on a general introduction article. We're in the "did you get the picture and is it decent" realm here.) The Olympus is also stabilized (via the sensor), while the Sony is not, which may mean that you can use slower shutter speeds (but not if there's subject motion).
That said, I've been shooting with m4/3 cameras for over two years now. From the beginning, I've found them functional, useful, and capable of all the image quality I need for many types of photography. With the right lens set, I'm not particularly afraid of low light. For my landscape work, I often use stitching techniques for large prints anyway, so I end up with a smaller, simpler system that I can carry further into the backcountry. Here's an example of one such stitched image (taken with an Olympus E-P2):
During those two years, I've posted a lot of shots without referencing what camera they came from. People weren't saying "oh, that must have been from one of those small cameras you've been using" or "that must be from you high-end DSLR." In the end, cameras are just tools. You learn how to optimize your use of the tool and you go out and create.
This is not to say there aren't limits. The m4/3 cameras, in particular, just don't cut it for me to perform star work (astrophotography). Nor are they always the right choice for me when I do wildlife work (I shoot an awful lot before dawn and after sunset, and with big lenses). But I use these small cameras for a lot of shooting these days, more than most people might have imagined.
The Bottom Line
Assess your needs. Assess what these cameras can do (I'll try to help with my reviews and other articles on this site). If the two things overlap, then welcome to the sans mirror club. If they don't, well, check out my other sites, as I have information about larger and smaller cameras there.