This lens was bittersweet to my wife and I. She is a portrait photographer and I mostly landscape. We shared this lens for some time, mostly her using it. What we found was that on the 40d there was inconsistent results. Sometime the focus was off, sometime the colors were not great, other times it was awesome. I'd say the bokeh at 4.0 was just ok, not like the 70 - 200 2.8 at 4.0 which has bokeh to die for. Our 50 1.4 at 4.0 kills this lens, not even comparable.
Then I had the chance to try this out on a 5d..BIG DIFFERENCE. Consistently better shots all the way around. Even the bokeh was better at 4.0.
After going through many many lenses on the 40d, the 30d the rebel and trying things out on the 5d, I've just come to the conclusion that not every lens that canon makes, even L's, is good on ever camera they make. The 17 - 40 was the same way, good but not great. Fantastic on the 5d and actually fantastic on the Rebel.
Buyers should take note of this when researching lenses.