Upgraded from a 10D and haven't looked back. This thing is incredible. For those who are upgrading from the 10d the first thing you will notice is the focus accuracy, finally a camera I I feel absolutely confident it in its focus accuracy. The next thing you will notice is very few blown highlights with this camera, the dynamic range just seems to be light years ahead of the 10D.
The camera is very well built and it feels really well balanced in may hand, its a bit heavy especially with the 24-70 attached but honestly I have carried it around for days on end on a trip to Portugal and I never felt that the weight was a problem.
I'm really impressed with the focus accuracy of this thing especially in servo mode, tracking fast moving object were always a problem with the 10D but this camera handles these situations exteremely well.
I know some users have complained about the battery weight and technology but quite frankly they are pretty amazing in that they seem to last forever. I usually get at least 1200 shots per charge and that includes a fair bit of chimping. I guess the only improvement I would like to see in the battery department would be a smaller battery charger, its pretty large and it's a pain to pack especially when traveling.
It take a bit of time to get use to the two button navigation/selection but after a while it really becomes second nature and it really hasn't been a really issue in all the shooting situations I have been involved in.
I really like this camera and I pretty sure it will be in my arsenal for a long time. Newer models will likely come along and at some point I will be hit by the upgrade bug but this thing will be around for quite sometime.
One thing I would like to see improved is the location of the FEL button, for some reason I find it difficult to reach/actuate, it just doesn't feel right.