The 40D's autofocus is quite good; I've had the opportunity to test mine in very dim situations and as long as there's good contrast, it locks on most of the time. I'd also be perfectly happy replacing my 20D with a 450D as a backup camera, if the batteries and memory were interchangeable. Unfortunately, when Canon made the move to SD it greatly reduced the likelihood of me (and probably others) ever buying the 450D at all as a backup. The battery design is also so new that I can't get any from sterlingtek yet.
I have NO appreciation whatsoever for a bigger, heavier camera over a smaller, lighter one. The ergonomics of my old XT never gave me any issues, even if my pinky was under the bottom of the camera. I can use small and large bodies, but it seems to me that smaller is simply more convenient and those people who love the larger bodies like to claim that "trait" as a boon. I disagree.
Small fits in tighter areas and is more subtle, both advantages for me. I don't care about impressing anyone with the size of my camera, even when doing weddings. If the 20D and 40D didn't suit my needs better than the rebel series, I wouldn't still own them. Believe me, I had a hard time keeping my 40D because I was unimpressed at the beginning, and I still have disagreements with it, but it does a good job and has some features very useful for busy, fast-paced work. If the autofocus didn't impress me so much I might've sold it.
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