Just got this lens, and I have to say, it's amazing! It opens up a new world to crop-sensor bodies.
At 10mm, you can achieve mind-numbing ultrawide landscapes, as well as those super-close cartoon-like portraits.
At 22mm, you have a great 1:1 (35mm) view. Truly a great range that compliments nearly any other all-purpose zoom lens. (think 17-40, 17-55, 28-135, etc).
My copy appears very sharp, and spot on USM focusing. I'm still testing everything, but it appears to be great.
Build quality is solid. Not quite "L", but this won't break on you. Price is fair, if a little high, but for what you are getting, it's definitely worth every penny.
This lens is probably going to stay on my camera most of the time, as I really enjoy wide angle shooting. Between that and my 60mm Macro, and 70-200 f4L, I'm very happy.