This was an upgrade from the 10D for me, so I was hoping that the improvements would make it worthwhile. After trying the camera out over the weekend I am very happy with the upgrade.
Although I preferred the size and feel of the 10D, especially with the battery grip, I am happy to sacrifice this for the benefits the 20D offers.
Autofocus - much more accurate and faster.
Frame rate - 5 fps is perfect for me.
Lower noise at high iso's - although some reviews have said that the noise output is very similar to the 10D, the fineness of the 20D's noise makes it much less visible. Even at 3200, shots were acceptable, and even more so when converting to b/w. Makes shooting at 1600 using ambient light very easy option.
ETTL 2 - Using the 550EX the result from flash photography have been excellent, even better results than I'd hoped for. The 10D used to sometimes give very erratic results.
Fast startup - You don't need anything faster than this. Reviewing pictures is also a lot easier, no more waiting for pictures to appear.
The new joystick control is also a nice feature, makes scrolling around images very easy. It does take a little time to get used to controlling the joystick though, but once you get the hang of it life gets easier.
Although the build is still good, the ergonomics, for me at least, don't feel as good as the 10D. The battery grip especially doesn't seem as good as that of the 10D. The 10D grip just seemed to be more solid.
Overall an excellent camera, the improvements are very noticeable and the picture quality is a definite improvement over the 10D. Definitely worth the upgrade from the 10D.