I got my 40D a couple of weeks ago and am very impressed with it. The camera feels solid and well made, and fits very well in the hand. The menus and controls are well laid out and simple to use. The viewfinder is nice and bright (though not up to 5D/1D standards), and the rear LCD is easy to see in most conditions. I've been really impressed with the battery life, the first charge gave me over 2000 shots!
My main reasons for buying the camera were the fast frame rate (6.5 fps) and the seemingly good high ISO performance. Neither of these have disapointed. The level of noise at ISO800 is so god that it often doesn't need NR (assuming you got the exposure spot on) and noise is acceptable at ISO1600.
The biggest and most surprising feature of the camera has been the image quality. I knew it would be good, but I'd not expected to see a noticeable improvement over the 400D. Images out of the 40D need less processing and sharpening to acheive the same results.
All round I'm very happy with the 40D (looking forward to trying it with the grip). My only issue is now I've got one I want to upgrade my second body too!