I just bought a 1d3 from another POTNer. I have barely warmed it up, but I'm extremely excited. My limitations with my current (not selling) 550D are AF speed and shutter speed. I was considering used 7D or well used 1d3. The 25% faster frame rate and seemingly much faster buffer clear made the difference for me. The AF between the two seem comparable, although some folks seem to have a love/hate relationship with the 7D focusing.
All-in-all, I'm not expecting individual favorite images to be much better on the 1d3, but I am expecting them to be more frequent, and much easier to acquire 
At any rate, I can't wait to get out there and start shooting. I have a family Tri to cover next weekend... I'm excited.