I bought this lens to be my walkabout on my new 20D. I was not expecting perfection, but overall I am very happy with my choice. Colour and contrast seem to be very good. Also, maybe I got a really good copy but I think it is pretty sharp.
At the wide angle it exhibits CA, but show me a Canon wide angle that doesn't. It is easily corrected in Photoshop.
On a 1.6 crop body this lens is a great all around performer.
I think some of the people who are down on this lens are expecting too much. If I want more I will use a prime or one of the L zooms.