Just got it yesterday!
I shoot a lot of classical concerts and recitals where flash is strictly forbidden. This means I have to rely on high ISO settings typically above 1600, and my 40D has been extremely frustrating with high noise results. Shooting in classical concerts with 40D has also been frustrating because of the canon-ball like mirror slapping sound it generates. In a very quiet venues, yes, 40D does sound like a canon-ball going off with audiences turning their heads in my direction with frowning faces on.
So, as soon as I popped in a memory card and a fully charged battery, I quickly tested its high ISO performances, and WOW, I mean, WOW!! These were shot with no post noise reduction of any kind, just in-camera noise reduction. These would look even better with the post noise reduction, I'm sure:
Prior to getting 5D3, I was wondering about whether I'd use the Auto ISO or not. With these results, I can rest at ease and leave it at the auto setting. How convenient!
As for the sound, I wasn't all that impressed with its silent shutter mode. Yes, the sound level has drastically gone down in comparison to 40D, and yes even from the 5D3's normal shutter sound. Perhaps my expectation was a bit too high, but it could have been quieter. I also didn't care for its not so "esthetically" pleasing sound. It just sound like a weakened mechanical noise, that's all. At least, its silent shutter mode would keep people's head from turning in my direction.
I also do a lot of birds (and drangonflies)-in-flight and sports action shots, as well as wildlife, so another thing that excited me about the new camera is its AF capabilities. I'll get around to testing the AF next time. My only hesitation in purchasing the new body was my concern regarding the "loss" of reach going from a cropped body to FF, but there were way too many benefits with the new body that I decided to deal with the reach when 7DII comes out, if at all.
So far, my only let down was when I found out that the RAW images out of 5D3 is incompatible with the CS4 PS and Bridge. I now need to either purchase CS6 or use DPP or to use DNG converting... all an extra hassle.