Well, I've now had chance to try out the new camera body on some paid jobs - which is, in my opinion is when I feel you really do start to get to know your way around it - you don't have a choice - you're on a job and the pressure to perform and take decent shots is on. You can practice offline with it and make sure you're aware of the basic changes since the previous model, but using on an actual job is where the learning really takes place.
First off, it's a great camera - I'd go as far as to say fantastic even. The leap forward in technology is quite astounding - however, there's so much that's different about this body that if I were Canon, I'd have created a new product family for it - it's just so much different from it's predecessor - menu interface - totally new and different, buttons in different positions, extra buttons with programmable functions. Apparently a lot of this has been taken from the flagship 1DX model - including the ridiculously complex (but ever so satisfying) focus system. The sheer amount of options you have with it are mind-boggling.
One thing that DID niggle me straight away, before turning the thing on was the new battery / grip they'd released to go with the new body. A step backwards from the previous one. On that one, you simply sled a level and the battery compartment lid fell open revealing the batteries, which just slid out for charging. In the new grip, you have to turn a small screw at the end of the grip and this then releases the entire inner cartridge from the grip with the batteries sat inside. And it's not incredibly robust - which I would have expected for the money you part with. You then have to take the batteries out of the cartridge to charge them. I can feel wear and tear taking it's toll on this long term. We shall see. (BTW, I had send the first one back, there was no battery cartridge in the box, instead a note, with handwritten message: "No cartridge at delivery", but still send it out to me. They sorted it out quickly enough and couldn't apologise enough. First time I've been let down by them though.
Handling feels very similar to the previous model, the new one being only slightly bigger with no noticeable increase in weight. The grip, for me, in essential - for a number of reasons i) I can grip it with my entire hand rather than feeling that I've just got hold of the camera ii) being able to shoot portrait and still have access to the main shooting controls without giving your self a crick in your neck and iii) a pair of batteries will take almost 2000 shots. I carry a spare pair of fully charged batteries and only even had to revert to them once on the previous model - it's just one less thing to worry about - to only downside is the additional weight added by the grip and the batteries - you just have to man up.
AutoISO is here to stay and I, for one, finally understand what all the fuss is about - I'm not sure in all honestly how I lived without it doing PR and events photography. Makes life so much easier. You can select a minimum and maximum ISO that the camera can use, coupled with a minimum shutter speed. In aperture priority mode, all you have to do is worry about framing the shot, choosing the right aperture for the effect you want, and, if things are within tolerance, the camera works out the optimal ISO to take the shot at the given minimum shutter speed you've selected in settings. No wonder wedding photographer rave about this feature.
Also, the body has far more control on the EX600-RT flash unit and ST-ET-R3 remote trigger - again, the new interface comes in to its own here - finding flash custom functions is a breeze and found it much quicker to change settings as and when the lighting situation changed. A big improvement.
The images the body produces are simply excellent. A modest increase in pixel count compared to the new DS and DS-R models capturing a massive 50k+ pixels - sadly, those bodies don't perform well in low light situations and so weren't for me. Having said that the quality of the images was very very good. A bit sharper then the previous model - seems to be better control over highlights and shadow noise. Speaking of noise - the 5D II was good at low light / high iso noise reduction, the 5D III is even better - partly down to the improved focus system, which almost seems like you can focus on a subject in near darkness!
In essence, I don't think there's any single aspect of this camera that Canon hasn't overhauled - hence, as I said at the beginning, it's that different, it could (maybe should) have had a new product name for it. It's not a 1D, but it's much more capable than a 5D Mark II - maybe 4D - anyway, they've called it a 5D Mark III...