I do not have this lens, but from feedback I have read on it, it is one of the sharpest kids on the block. Really good reputation. I have the Tamron 28-75 2.8 and love it, great sharpness and noice colours, so I think you will be more than happy.
Just on this note, I have a 450D and have sold my Canon 50mm 1.4 (used it extremely rarely) and also sold my Canon 17-85 IS lens. I was going to purchase the Canon 18-200 IS lens, but I’m kinda leaning towards the Canon 17-55 f/2.8 (or Sigma equivalent) or the Canon EF 24-70 L.
The camera is used for all types of shots (walkabout and travel mainly) plus family events. I’d like the f/2.8 because I may have a chance of being 2nd shooter at a wedding at some point and I’d like the experience. Plus, the Canon 17-55 has had great reviews.
Freddie,
Tamron is better than sigma’s equivalent (18-50/2.8) if we are talking about optics. Sigma’s lenses are much more robust though.
If you can afford canon there is no question – go for it, but it doesn’t offer much more resolution than tamron. And IS is not important for such short focal lenghts. Don’t let it bother you.