I'm seriously considering moving to micro 4/3s for my main camera now that the GH2 is up and coming, here's why:
1. To get a larger viewfinder than my 40D I need to go to the 7D or 5D, which are $1,700 and $2,700 body only. The flagship GH2 is about $1,000.
2. Size/weight. I actually like a camera the size of a digital rebel, but have stayed a model up because of the larger viewfinder and better body controls. The GH2 is a perfect balance of small, with good grip and button placement. The GH1 wasn't bad, but the wheel on the front meant you had to take your finger off the shutter to change settings - much improved with the GH2.
3. Mirrorless. This isn't an advantage per se, but with the GH2 they have solved most of the negative, and added a bunch of positives:
3a. EVF. While an EVF isn't as good in some situations (low light) they really are pretty darn good, and the GH2 offers twice the refresh over the GH1. It also offers the same size view as a 5D in a body smaller than a rebel!
3b. Mirrorless. By getting rid of the mirror the camera functions the same way regardless of whether you are using the rear screen or the viewfinder. With the Canons you are still changing from great phase detect AF to crappy slow contrast detect when you switch. Only the 60D offers a flip out screen, and still a more expensive body.
3c. Sensor performance. Too early really say on the GH2, but with my GF1 I did some testing and an HDR capture made with it looks fantastic as an 8x10, which is as big a print as I'll ever scrutinize. I mostly put photos on my blog or in a photo book, so nothing bigger than 8x10 will be viewed closeup. Further, I rarely shoot at high ISO, and if I do I'm happy to accept some grain. Bottom line is that the dynamic range of these smaller sensors is still as good as my 20D was, with lower noise. Wasn't I happy with the 20D when it came out? ;)
3d. I'm able to get stock photos accepted from my GF1 at about the same rate as my APS-C camera, and if ever there was a test of performance viewed at 100% on the screen it is getting stock photos accepted.
4. Because Panasonic has embraced 4/3s as their only sensor in both still and video camera, I look forward to their plans for dedicated lenses. Unlike Sony, and probably Nikon and Canon, they are making lenses for full frame, APS-C and micro - which means most of the really good lenses are way bigger than needed. All my nice L series lenses for my 40D are wasted to some degree - heavier and more expensive as a result.
5. There are some very cool features that Panasonic offers because they have embraced this format with no legacy equipment: 3D lens (which presumably they correct in camera to only show the view from one lens), touch AF, AF anywhere on the sensor, re-sizable AF points, face detection AF that is fast, cropping in camera at 1:1, 16:9, 3:2 and 4:3, live black and white, 40fps crop mode, 2x digital zoom in still and video.
6. Video. I'm not a huge fan of taking the time for video, but at least this camera offers some advanced stuff like full manual control, AF during video, audio levels and a flip out screen. It also offers some of the highest bit rates in a combocam, plus video out monitoring. Video isn't going away.
7. Easily adapted lenses. Using the very short mount means any lens can be adapted to this format. Some cool stuff like tilt shift lens adapters are possible too.
8. I'm trying to get out of the constant upgrading of equipment. With micro four thirds I can get more features, same lens coverage in a smaller kit and end up with money left over - versus dropping another $2,000 for a state of the art Canon. No focus micro-adjust needed, either :)
What am I losing?
1. Fast lenses. This isn't a big a deal - with my Canon my L zooms are f/4 anyways, and the 20mm f/1.7 is my perfect focal length. My guess is that Panny will eventually step up with more options.
2. Macro. There aren't a lot of cheap macro options.
3. High ISO - but I hardly ever actually take advantage of it.
4. Weather sealing and durability. There is no question that my L lenses are tough, but they are also really expensive. I can replace a lot of 20mm pancake lenses before I'd pay for a 35mm L.
5. Depth of field. This is a wash - with wide angles you can get more depth of field at say f/2.8. With the larger sensor you should get more background blur - but I don't notice this in practice. Being selective about what is behind your subject makes more of a difference. I don't own a f/1.2 lens for my Canon, and frankly my 20mm f/1.7 can be used wide open in bright sun - not realistic with my 28mm f/1.8 Canon.