I'm a keen amateur, using a 10D for a few years now, waiting for the right time to move to a 1-series for better sports shots, mainly soccer. I was planning on buying at B&H during a visit to NYC at the end of October but ended up getting a surprisingly good deal in the UK before my trip, and took the plunge with the AI issue looking as if Canon were about to get to the bottom of it.
The first thing to strike me was just how easy the camera is to use for someone familiar with the 10D. Although the button layout is a little different the logic is very familiar and I felt comfortable very quickly. The second thing was the quality of the pictures; I genuinely didn't expect the shots to be noticeably better out of the box - I thought focus tracking would be better and obviously low light shots would be improved, but just your everyday point and click shots are that much sharper too.
As an example, I'm a sucker for clicking away at seagulls. Alongside the Libert Island ferry, with the boat pitching around and a non-IS lens, every one of a dozen or so shots is perfectly sharp. I haven't suddenly become a decent photographer, and the lenses are the same, so I am confident that the improvement over my usual soft efforts is down to the camera. Some of the shots from the Empire State Building, at night, ISO 3200 and 6400 and handheld at F2.8, sold the low-light performance to me.
I spent a morning calibrating the lenses with Art Morris' excellent mkIII user's guide, and found every lens needed adjustment. My 85mm 1.8 is going into Canon this week as even at +20 (the end of the scale) it still isn't perfect - that would at least partly explain why I never took a good picture with it then....
After a week spent using the camera in NYC I came back with a stack of pictures, a far higher percentage of which than normal I consider keepers, and a fairly sore left shoulder from carrying my camera bag pretty much non-stop!
From an amateur's point of view, to anyone looking to make a similar upgrade I would say go for it - the 1DIII is simply a fantastic piece of kit, and the difference in output between it and the 10D more than justifies the cost.