p.2 #3 · Sigma 17-50mm F2.8 vs. Tamron SP AF 17-50mm
I bought the Sigma 17-50 f2.8 OS last month and use it on a 60D. It's sharp wide open and just gets better as you stop down. Results are really close to my 70-200 f4 IS.
I tried Canon's 17-55 IS a couple of years ago but found it just a bit too big and heavy for my everyday setup. The Sigma is more compact, lighter and saves you $400.