This lens blows away every other lens I own, hands down. In fact, I put 4 lenses on eBay within 3 days of receiving this lens (a Sigma 24-70 f/2.8, a Sigma 18-125 f/3.5-5.6, a Canon 35mm f/2 prime, and a Canon 24mm f/2.8 prime). The difference in sharpness and detail of the pics taken with this lens vs. these other 4 was astounding to me - even between the primes and this lens. If I hadn't seen the difference with my own eyes I'm not sure I would've believed it.
I read somewhere where someone said he/she doesn't have to sharpen the pics they take with this lens. I didn't really believe that when I read it, but I do now. And, I don't have to sharpen my pics with this lens, either.
Yeah, it's a tad heavy and a tad bulky, but those are very small inconveniences when compared to the quality of the lens and the pics it allows you to capture. All I can say is if the pic(s) I take are sucky it's not because of my lens or Rebel XT - that fault is gonna fall squarely on my shoulders. ;-)