If you get a good copy of this lens, it is really fantastic. I've had two. One of them, which I bought for my school and don't get to use anymore, has great IQ, color, contrast and sharpness -- all for a fraction of the price of other f/2.8 general purpose lenses. The second one I bought (for myself personally) stunk it up. There was some sort of major focus/sharpness problem, and I promptly returned it. I believe others have had similar experiences with this lens.
If you buy from a reputable dealer with an easy, "no questions asked" return policy, I wouldn't hesitate to give it a try.