Rented one years ago and used it on an XTi on a trip to see polar bears. Worked pretty well on that body even at dawn and dusk. Focus was reasonable. Clearly (wow, a pun) the 2.8 variety is a stellar lens but the f/4 is pretty darn good for the price.
Ah. I misspoke. I found the files and this was at 6:53 am in November near Churchill, Manitoba. It was still pretty dark and this was handheld at f/4, iso 1000 and 1/500 sec then cropped. It was on my 50d, not the XTi. I know the color balance could be better and I have since learned a lot about better post-processing. But, the point is that in existing light in the dawn hours handheld using a camera that had a lot of noise, this lens did pretty well. I hope that gives you and idea of what the lens can do and in better light, this would be a nice shot. Maybe someday I'll repeat the trip with FF and the f/2.8!