haven't checked this one out, but 18-50 is a good range, and f/2.8 is solid too. I have the Canon 17-55mm f/2.8 IS USM and I love it, so if the sigma is anything like that then it should be great.
Another lens that I would strongly recommend is the Canon 50mm f/1.4. Even though it doesn't zoom or anything, it's supposed to be one of the most 'universal' lenses and falls within your price range. I didn't buy one when I bought my T2i and I regret it.