I switched from Nikon to Canon (am I the only one doing this too LOL) last spring before wedding season and did some research online... Really dug the specs on the 5D3 as well as the 600EX-RT flash system.
I made the jump and sold my Nikon kit which consisted of a D3s, D700, 3x SB-900, 24-70G, 70-200VR1, 14-24, and 50 1.4 I ended up buying a 5D2, 5D3, 5x 600 EX-RT, 35L, 85 1.8, 135L, 70-200L IS1, and 17-40. I really haven't looked back since and shoot 85% of the time wielding two bodies with the 35L and 85 1.8.
Here is what I love about the 5D3
The 600 EX flash system has changed the way I shoot, at any point during the day of a wedding I will have at least one additional light firing off-camera. What a wonderful system. It works flawlessly in group mode when I want to light up a reception hall with 2-3 lights. My only gripe is that it doesn't work the same with the 5D2. Canon claims it is the processor, but I think they just want to sell more new cameras
The 5D3 files I have shot look indistinguishable if not slightly better than my old D3s as well, which is a blessing given the smaller size. The ability to shoot smaller RAW files has also been a charm. During the reception and other boring parts of the day, I shoot away in mRAW as very few wedding clients need a 22MP file. Despite a smaller file size, I like to perfect WB and exposure (which usually needs very little tweaking)...
The focusing system is incredible in my opinion. People complain about it being confusing, but once you sit down and really put all the different focusing patterns and modes into practice, I rarely get an OOF shot even with a prime at ~f/2. Granted my keeper rate is much higher with the 70-200 f/2.8 IS, but I think that is just a function of focus speed with primes vs zooms, not anything to do with the camera. I did notice the camera focused a little slower with the 600EX AF assist beam, so I did a little test. I have my DOF preview button set to switch between SERVO>ONESHOT when I hold it down. I noticed that the camera focused much faster in SERVO mode without the AF assist beam (which it doesn't use anyway) even in very dim lighting. Now even in ONE SHOT, I only use the AF assist beam when it is absolutely pitch black which is almost never these days since video guys always have their annoying video lights on heh. From my experience using zone focusing/tracking, the 5D3 tends to get "thrown off" by objects in the background much less than any of my Nikons, even the D3s.
Perhaps the one "awful" thing about the 5D3 is the dark AF points in low light. My Nikon cameras always had a red square and it made moving AF points around in low light a breeze. I have learned to adapt to how my 5D3 works in this regard, but I am hoping for a FW update to remedy this at some point. Not keeping my hopes up, but it would be nice.
As far as dynamic range, maybe my exposures are just really nicely lit because of how often I use the off camera flash system. I don't really ever need to push or pull my RAW files more than 1EV, so maybe I'm not seeing this lack of dynamic range so many people have complained about. There are certainly merits to having more dynamic range, heck I wouldn't argue against it. For landscape and studio shooters, I'm sure the D800 is a better choice, but right now the 5D3 is the best camera I have ever used for a wedding, right ahead of the D3s.
So overall I really have nothing bad to say about the 5D3. I view it as a baby D3s. I shot just over 35 weddings this past season and it took only a couple jobs (second shooting for another studio in April) to get fully accustomed to the Canon controls. I actually prefer how ISO is changed on Canon bodies and I never even have to take my eye away from the finder. Despite some minor quirks here and there, the overall value of the 600EX-RT system more than makes up for anything I could complain about.
Am I just oblivious to problems or a lot of people just suffering from "the grass is always greener" syndrome because of the rave reviews the D800 gets on websites like DXO?