I've been using this lens for the last year and have found that the AF can be a bit difficult to work with in situations with low light or low contrast. However, for the price, it is a good buy and generally performs well with a little coaxing.
Part of the problem is comparing the AF on a lens like this to more expensive models. It's perfectly fine unless you are micro scrutinizing the performance. I always get great shots in real life--which is where most people shoot unless you are going for pictures of little black and white lines. =}
Having used all three 50's, I've found that of the three, the best cost-for-performance model is the 1.4. The improved AF on the L is not worth the extra coins for me.