I've had my 40D for a couple weeks now, and I'm impressed. The up-grades from the 30D are not just minor tweaks. When the 30D came out, a lot of us were disappointed as it was little more than a 20D with very minor improvements. This is not the case with the 40D. The improvements are significant and very noticeable.
Perhaps the biggest is the new AF system. On paper, it doesn't sound like it would be that much of an improvement over the 30D. In reality, it is a significant improvement over the 30D. In fact, the improvement is so significant that reports are coming in of photographers finding they can use lenses with their 40D that they couldn't previously use with their 30D or 20D due to focus issues.
I don't personally have any lenses that don't work with my 30D or 20D, but a close friend of mine has 5 lenses that had focus issues with his 30D that are now working perfectly with his 40D. Does the 40D have AF Super Powers? Who knows. I can say that it works wonderfully.
My 40D is paired with my Canon 17-55 f2.8 IS pretty much 100% of the time. My 30D, 20D, and XTi usually have a fast prime or longer zoom on them, depending what I'm shooting. The 40D / 17-55 f2.8 IS is a KILLER COMBO! More and more, I'm shooting almost entirely with it. Focus is very fast, images are sharp, and I have virtually no OOF (out of focus) shots.
Even in dim lighting, the 40D & 17-55 just kick butt. I shoot a lot of weddings, and most receptions end up with poor lighting by the time the dances get going (if not before). The 40D & 17-55 have been amazing in these situations. I'm getting shots that the 30D would not have got, and the focus accuracy is greatly improved in comparison to the 30D.
The AF improvements of the 40D are so significant, that I'm putting my money where my mouth is. I originally planned on adding only one 40D to my current camera line-up. After shooting with it for the last couple of weeks, including two weddings, I will be adding a second 40D, if not a third. The 40D is just that damn good!
That said... if anybody is interested in a 20D in excellent condition, send me an email. :-)