I had Canon 450D for 3 months (got almost the day it came out) and now with Canon 40D - so I can say as a user of both. *A lot* has been said already, but several important points (IMHO):
- In ~70% of my shots I would get
the same image quality on both cameras (with the same lens, settings and conditions, of course)
- In
every other category - Canon 40D wins hands down. This is expected as Canon 40D is positioned as the next level from 450D (or any Rebel).
The first point makes 450D an awesome buy if you own just one lens and primarily care about casual shots coming in a nice quality. This is about as best image quality you can get in any crop dSLR (including Canon 40D). There's practically nothing on the market from Nikon or Olympus or Sony that can beat this quality for a price.
Canon 40D, being slightly better in image quality in some situations, excels in every other category - for the extra ~$300 (at least in US right now).
Hope it helps,
Nikita.
http://www.flickr.com/photos/agilepixels/