This is an amazing lens ... I've had it almost all the time in my 5d mkiii , been neglecting my 17-40mm and 70-200 f4L's due to their nasty weight. Been shooting strobist and natural light and am loving it, you can see my shots in my website.
One issue though is that my copy seems to be front-focusing, I fixed it by using 5d's micro focus adjust. However it's unfortunate for my 60d not have one >.< , I will send this to Canon soon.
Another issue is to be careful not to apply pressure at the lens front, since it extends like a 50mm 1.8, it will simply stop focusing if held with pressure - a quick fix is to remove the lens (or battery) and put it back again. A permanent fix is by applying the newly released firmware.
Enjoying the lens so far, I hope Canon makes more of these soon!! :D