I've had this lens for a month and while it is certainly sharp, it does have issues. I also own the 50L and have for sometime and I'm very familiar with the 50L's quirks.
The 50A does not like BBF in AI Servo focusing method. My experiences have determined that using One Shot and hearing the focus confirmation beep is the only way to confirm locked focus. In addition, AF should be setup for single AF point and not surround. I don't focus and re-compose but I can imagine that this technique could result in inconsistent focus.
Given what I know about this lens would I have bought it in the first place? Probably not. Too many work arounds and more than the 50L. Build quality is great, looks nice, nice case but all these things should have been worked out by Sigma before release.
This is my first and likely last Sigma lens. Hoped my experiences would have been better and probably would have bought the 35A. Not going to do that and i'm thinking the 50A will go on the block in the new year. YMMV