This is one of the main lenses for wedding and event photographers. It's rather large lens and has an even bigger hood. For some reason it seemed disproportionate the the lens size.
It does everything that you'd expect from Canon's "L"uxury line. It's gives you good colors, sharp images and is built like a tank. It gives you a constant f/2.8 throughout the range so it gives you a little more flexibility in the low light department. Optically is a top performer, but...in the end I thought the range was a little short and the wide end wasn't wide enough on a 1.6x body. This is truly a wonderful lens, but I think there are better options out there right now especially for EFS mount bodies.
Here's a few photos taken with the 20D.
63mm, 1/400th, @ f/8, ISO100
27mm, 1/400th @ f/6.3, ISO100
50mm, 1/400th @ f/7.1, ISO100
28mm, 1/80th @ f/5.6, ISO400 (SnS, you'll recognize this one)
54mm, 1/30th @ f/10, ISO800 (This one was shot at a higher f-stop to increase the dof)