I have been the owner of a Canon EOS 5D Mark III for about 18 hours. Of those 18 hours 8 were spent sleeping but that doesn't mean that I can't have an opinion already :-)
So far so great! Anyone who is used to the sound of the Canon EOS 5D Mark II's shutter firing will instantly fall in love with this one! It sounds, dare I say, Nikon-esque?
Ok, enough silliness for now, let's get serious.
Ergonomics:
What a difference (in a good way). As soon as I took it out of the box I noticed how differently it felt in hand. I haven't handled a 7D very much but I think they took some cues from that body. A small thing for most I'm sure, but they moved the DoF preview button to what I feel is a much better location. I was constantly hitting it by accident (triggering my 430 EXIIs to go into "modeling" mode). I would always hit it by accident but to hit it on purpose almost seems impossible now. Now I can quickly hit it with my right ring finger with ease. Many other buttons and switches seem similar enough to me (also in a good way). The way you zoom in to view a picture on the back of (the new and gorgeous) screen is quite different. I can't stop hitting the AF point selection button for zooming to no avail. I am sure I will retrain my brain to adapt to the new way.
Auto Focus:
Wow! I will have to read the manual about 6 more times before all of this new technology sets in! There's a lot going on!!! For me, the most important thing is that I now have more than just ONE cross-type focus point, and for that matter, more than ONE "usable" focus point. OK, that's not completely fair but in low-light the outside focus points would hunt more than anything. I am actually a big proponent of "focus and recompose" but it's not always practical. Further more, we have moved from EV -.5 all the way down to EV -2. That's going to be huge in dimly lit churches and reception halls (not to mention my family room, er cave!). Testing both the one shot and servo varieties will take a while to fully vet out but I am liking what I am seeing so far. Which brings me to a question: For those of you that already have experience with it, do you favor "spot" over general manual selection? So far the jury is still out on the this one but again, it's going to take me a while.
Menu:
Different but similar. I found the old one a little quicker to navigate because I found it easier to skip over menus rather than scrolling through them sequentially but that may just be something I need to learn how to do.
AE:
Seems to be a little better. I always find that Canon's AE is about 2 stops under exposed for me in bad light. Shots I've taken so far are only 1 stop under exposed. I have had pretty good success in good light (light coming from behind you). I tend to shoot in "M" but it's good to be able to shift to AE when you need to and feel pretty good about it. I hope to test this one thoroughly. As long as it's consistent you can use EC. On older bodies I have not found that to be the case though.
ISO:
If I had to guess I'd give it a full stop over the Mark II. I did take some shots last night at 12,800 and out of the camera they looked pretty amazing! I was in a cavernous room only lit by a TV and both auto focus and high ISO performed greatly. I should add that the Mark II was able to lock focus rather quickly using the center point though :-)
Speed:
Love the 6FPS and more so the much reduced shutter lag. This camera is responsive!
Feature Set:
Love the dual card slots. I shoot redundant to the SD card to make me feel warm and fuzzy :-) The ability to set MIN/MAX values for ISO and shutter speed is a great add. Oh, I almost forgot! LOVE the lock feature. Between that and the locking mode dial I should have much greater success when shooting with two bodies on BR Straps and wondering why I am on "B" mode when shooting portraits!!
The Truth:
Out of the box you are not going to see much difference in an image taken with the Mark III vs. the Mark II. There is a lot to learn before you can harness all of the power of this new body. Once I know this body inside and out though my expectation is that my hit rate improves measurably.
So, what I would like to do is to revisit this after I have had sufficient time to learn about the Mark III in practice. I would also like for other owners to add their thoughts, opinions, etc. What do you like vs don't like? What help can you offer? What have you found buried in the menu that someone else may be searching for?
If you've stuck with me for this long you are a much better man that I am! I spaced out 4 paragraphed ago :-)
***SHAMELESS PLUG: PLEASE BUY MY Canon EOS 5D Mark II's :-)