I just spent the last month going through numerous lenses for my 20D. I'm a serious amateur (if there is such a thing) and I was trying to get one lens that would do it all- 18-200 and 24-200 as well as some 28-200's. The thought was I'd invest in the higher end lenses later. Well, I went through Sigma's, Tamrons, and even the highly rated Tokina 24-200 (which after 200 shots and tests against the kit lens showed it barely better) as well as some of the less expensive Canon lenses. I finally had enough and forked out the $ for the low end of the L series.
Wow- it blows away anything else I tried. I will say that I am using the Tamron 28-70 2.8 for the mid range zoom and am very happy with it too.