I got a (refurbished) one a few months ago and I've got nothing but praise for it. USM focusing is lightning fast and super quiet, and if you're outdoors, wide-open and ISO 200 will give you fantastic shutter speeds for capturing action while having a little subject isolation. The image is razor sharp wide open (although I'm sure the f/4 IS is even more so) and I haven't seen any CA in my occasional pixel peeping. The size, if you're used to consumer lenses, is a bit unwieldy and unbalanced on a Rebel body. Lack of IS is notable, but you can bump the IS or get good at handholding, I suppose.
I've handheld at 1/60, 200mm with some success, but it takes practice.
It may be Canon's cheapest L lens but it sure doesn't skimp in IQ and awesomeness!