Generally, the 10x or greater zoom lenses are not very good in image quality, they tend to be slow in focusing, and their maximum and variable apertures can be frustrating. Their only advantage is the convenience of their focal range.
What is your budget for lenses? Personally, I think you'd be better off with a two-lens solution. The Tamron 17-50 f/2.8 gets great reviews, and is about $450. This would give you a large aperture lens for lower light shooting.
For a telephoto, the Canon 70-200mm f/4 L and Canon 70-300mm f/4-5.6 IS are both around $500, and are excellent "bang for the buck" lenses. The former has professional build quality, faster focusing, but less range and no image stabilization. The latter has image quality close to the "L" lens, an extra 100mm of reach, but does not focus as fast, and has consumer-grade build quality. You may be able to find a good deal on a used 70-200 f/4 L, since a lot of people are upgrading to the new version with image stabilization ($1,000). Check out the buy/sell forum on
www.fredmiranda.com .
When I bought my Rebel XT, I started out with the Sigma 17-70mm f/2.8-4.5 macro ($350) and Canon 70-300 IS lenses. The Tamron 17-50 I mentioned above was not available at that time. I've since upgraded the Sigma to a Canon 17-55mm f/2.8 IS ($950) and added a Canon 10-22mm wide angle lens. I still own the Sigma, but haven't used it since getting the Canon 17-55 IS.
Also, if you want to experiment with prime lenses, the Canon 50mm f/1.8 is a very inexpensive way to start. It's not well constructed (has a plastic mount), but at $80 it does have very good image quality for the price. I own this lens, but rarely need it now that I have the Canon 17-55 f/2.8.