The Canon 17-55mm f/2.8 IS is a better lens than Tamron. The most obivous advantage is the IS, which is very important in low light photos. For a lens with f/2.8, it shines in low light photography and the IS will be a must have if you plan to shoot handheld. You will need to carry a tripod with the Tamron. Most lenses with IS will cost about $600 extra. But it is definitely worth it for me. Canon is also optically better than Tamron, and there is also some minor concerns of some people about using 3rd party lens and compatibility issues.
I have the Canon 17-55mm f/2.8 IS and love this lens. I use this lens even more often than my EF 70-200mm f/2.8L IS. The image quality is L class. Here are some samples of my own photos using the Canon 30D and the 17-55mm f/2.8 IS lens. I'm only an amateur and all the photos are taken handheld.
Here are some photos using both the 17-55mm f/2.8 IS and the highly regarded 70-200mm f/2.8L IS. This is just for comparison, with the same photographer using the same camera body. The image quality is very comparable.
I have not used the Tamron lens, but I can tell you that the Canon 17-55mm f/2.8 IS is an excellent lens with high image quality and performance even in the hands of an amateur like me. It is also fast in low light and a pleasure to use.
A couple weeks ago I got great advise about a wide angle lens for my new Canon 30D from several people on this forum, thanks to all.
I have narrowed it down to 2 lenses.
I ran into a guy in Reno who has a Canon 20D and had a Tamron 17-55mm f2.8 XR di-II lens and the pictures he was taking were excellent. I looked that lens up when I got back and it sells for $409.00 locally.
I am also looking at the Canon EF-S 17-55mm f2.8 IS USM lens which also sells locally, but for $1029.00.
The question now, is the Canon lens worth the extra $600.00?
The money is not a big issue now as I was very lucky in Reno and the Casino will in a way be paying for it.
Any advise will be very much appreciated.