I strongly disagree with some of the reviews here that trash this lens especially since they almost dissuaded me from buying it.
I just bought a 7d and got this lens for it. I considered a variety of L lenses and the Canon efs 17-55 but ultimately choose this based on all the positive things I had read and the crazy price.
Initially I thought it wasn't so sharp until I realized I was seeing motion blur or camera shake or the focus was off. Once I started to get more familiar with the camera and figured out the settings that worked for my shooting style, I started to see extremely sharp results. In fact, I was seeing pics of my son taken from 4 or 5 feet at 17mm where i could literally count the eyelashes.
Furthermore, I was able to compare it to Canon's very expensive 16-35 (or whatever the long focal length is) taking shots of the same subject in the same light and position on the same camera, and to my suprise, at the widest focal length the Tamron was quite a bit sharper. There is a small possibility that something other than lens sharpness contributed to this result, such as camera shake since i didn't have a tripod, but I couldn't find any sign of any. Whatever the case, being anywhere close to a lens that is many times more expensive is pretty good value.
I have also had some trouble getting focus locked in low light, but then it also happened with the Canon. The difference is that the Canon searches quietly whereas the Tamron screeches CANT FOCUS as it travels through its focus range. This is acceptable to me unless i was in a church in the middle of a wedding ceremony. Of course for this price there has to be compromise somewhere and I'd rather have noise than fuzzy pictures.