If you haven't already noticed, most of this site is a review of this particular lens. It rarely leaves my camera, and for good reason, but as anyone who has spent any time with lenses knows, nothing comes without sacrifices. I'll start with the cons so as to not waste your time if any of them are deal breakers for you.
Cons:
This is a prime lens, as in not a zoom lens, as in no zooming.
Tunnel vision. a 25° angle of view means that it pretty much sucks for landscape photography, and to take a big group portrait you have to stand back pretty far.
It is optimized for 1.6x cameras like the 20D-50D and the Digital Rebel series, which is a plus if that is what you have, but it wouldn't function right on a full frame camera like the 5D MarkII
Hmm, well, I'm out of cons already. On to the Pros:
The lens doesn't change length when you focus. No bumping that bug off the leaf as your lens pokes out, and it lessens the risk grit and dis-alignment.
The focusing wheel is big enough that you don't have to hunt for it, and is built really well. It is coarse enough ...