I currently have a 50mm f/1.8 and I'm really struggling with the AF. It's awful. I don't know if mine is screwed up after a week of shooting in the desert or if it's just screwy but i end up manually focusing at f/1.8 frequently which is a bit of an adventure especially with the microscopic focus ring on the 1.8. My question is the boost in AF performance worth the 200 or so bucks in the upgrade to the 1.4 USM? I'm a college student so the 1.2L is out of the question in terms of cost.