Originally Posted by Sean Setters
I own the Tamron17-50mm f/2.8 XR Di IIand its a really really good lens -it's the way to go in your price range. You can pick one up used (i paid $308 for mine used on ebay almost in new condition) for under $400 (or new for $450), constant f/2.8, an its just as sharp as the $1000 canon 17-55. Its really the way to go and a great general purpose lens on a 1.6 chop camera. For the money you just cannot go wrong.
The things i dont like about it is its a bit loud (noisy) focusing, and its not as fast focusing as some of the nicer USM Canon lenses but it does ok. Also it doesn't have IS but its such a fast lens you don't really need it IMO.
They do have a VC (Image Stabilization) version for around $650 but i don't think its worth the money and i haven't read enough reviews to know if its any good or not.
Tamron 17-50 @ f/2.8 ISO800 on a Canon xti
http://liam821.com/pix/other/IMG_8511.jpg <-- high quality version