I've owned the 40D since it was released in 2007, and while it does have its limitations (poor ISO performance at 800 and higher), I really love it. Its strengths are build quality, frame rate, and image quality at ISO100-200.
Build quality: alloy frame, some weather sealing - although when shooting in the rain a couple of times I've had some of the controls cease up on me. (Canon has never suggested that it could be used in such weather). It's always come back to life after a bit of drying, and the whole camera still looks like new after five years.
Frame rate: 6.3fps is a nice frame rate for the types of photos I like to take: triathlon events, kids running around, wildlife... the AI servo mode suffers a little bit because of the camera's weak AF system though (but I'm comparing it with my lovely 1V, which isn't really fair).
Image quality: at ISO100-200 this camera produces really nice clean images that can be blown up reasonably large.
It is still my main digital camera, and it's only now with the release of the 5D MkIII that I've seriously considered replacing it. I almost bought a 5D mkI last year (to use for 'wider' angled shots), but it felt so *old* next to my 40D that I spent my money on a new 10-22mm EF-S lens instead (which was the same price as a used 5D).