Now that Canon has introduced their mirror-less camera system, we have entries from each major brand:
Canon are the newest with their EOS-M.
Pentax have their K-01 and Q-system bodies; one huge, one tiny.
Nikon have a small format with the distinction of phase detection auto focus.
Samsung have had multiple models but not much success.
Sony have had multiple models and some success but lack good lenses, or many lenses of any quality.
Panasonic and Olympus originated the idea and have a diverse selection with some excellent fixed focal length lenses, but they're struggling with size and weight to bring excellent zoom lenses to the market.
Comparing them casually
To me, Canon's entry is only adequate. They're coming to market with a mediocre set of specifications and features. The image quality is quite nice, but they waited so long that you're paying a premium price for performance that's already been done.
Pentax have the most bizarre ideas for mirror-less cameras. The K-01 is about the same size as their dSLRs and mounts the same lenses, but it also has all the disadvantages of mirror-less cameras with slow operation, and of course, the most unusual designer design. The Q-system isn't really winning any friends. It's small but it's not cute. They could have designed it to match the old Auto 110 SLR (they had a point-and-shoot like that) and it would give people a fond memory on which to purchase the system. Instead, they gave it a blocky design that looks uncomfortable to hold and use. Supposedly, it doesn't work all that well, either.
Nikon have done well in selling their 1-system. I suspect it's mostly their dSLR customers buying into the system but it's small and pocketable and cute. They're on their second generation, and it's not as bad as it might seem from the specification.
What happened with Samsung? They couldn't get along with Pentax? They had worked together as Samsung got into the dSLR field but they just couldn't find common ground? It seems as though they copied Sony style and specifications (and name, NX vs. NEX?), but came up with something almost no one really wanted. I've been told that their dSLRs were better than the Pentax branded dSLRs from which they were originated, so why couldn't they work with Pentax so both companies would avoid embarrassment?
Sony keeps coming up with clever bodies to match their electronics focus. Unfortunately, they made most of the bodies so compact that you need proprietary accessories. Besides that, the lenses required by the image circle or the APS-C sized sensor have to be rather big and appear huge on the smallest bodies. Image quality is generally good, but the compromises are high for what they're asking of the user, especially since they have few good lenses and no spectacular lenses.
I'm more inclined to believe in Panasonic and Olympus since I'm a Four-Thirds user, having two high end Olympus dSLR bodies. They have a number of fixed focal length (prime) lenses that are quite great. I keep reading that they're even better than those on 135 format camera bodies. It wouldn't surprise me since Olympus has a history of great lenses. The zoom lenses are the problem. Both companies have chosen small and light over excellent image quality. Panasonic often corrects lens problems in the camera's firmware and Olympus has begun to do this also. This still doesn't excuse poor designs but for hobbyists, does it matter? I see a lot who want a US$5000 lens for US$100 but I suppose that's always the case. What's sadly humorous is that I cannot nitpick the others like this--they don't have the products to nitpick. The micro Four-Thirds image quality has improved so much that many reviews of the Olympus E-M5 suggest that you need to move to 135 format (full frame, as they mistakenly call it) to see an appreciable difference.
In 10 years, will there even be a dSLR market?