So far this is the only lens I have for my 5D. I read reviews that said it was a high quality lens for the price, and although I don't have any L-series lenses yet to compare, I have to say I love this lens.
Glad I Bought:
I bought one of these and have no regrets. It takes two camera batteries, which doubles your shooting time :) It comes with a AA cartridge as a back-up, which is the main reason I bought it. An added bonus is the additional shutter release button - whether you're shooting vertically or horizontally, your hand is in the same place (i.e. no awkward grip).