This lens, like all other lenses, is a compromise, and as such, it does a great job for what it is designed to do. Anyone who tries to compare this lens to other lenses that are NOT in the same class or kidding themselves. I have tried several lenses in several mounts, and I have to say that this lens fits my personal needs very nicely, and even in prints up to 12x18 inches you would be hard pressed to tell which lens/camera combo took which photo.
But here is the bottom line. Yes, this lens does not focus as fast as a USM/HSM/AF-S lens, nor is it as quiet. However, the 18-125 (28-200mm equivalent) zoom range is awesome since it reduces the number of lenses changes I make on my 20d, which results in less sensor dust and fewer sensor cleanings.
I plan to supplement this lens with the 10-20 and 135-400, giving me an effective focal range of 16-640mm in only 3 lenses!
Considering the price, the build quality is good, and I feel like this lens is a significant improvement over the 18-55 Canon "kit" lens, if not entirely from an optical standpoint, from a complete package standpoint it is an excellent alternative - and I even like it better than the 17-85 USM IS lens I previously owned.