This is the best lens I own, bar none. I hear some people have issues with autofocusing this lens, but I rarely have any. (I do often use an assist laser so maybe thats part of why?) Focus is fast and reliable on my copy.
Only downside I can think of is the weight. It is much heavier than it looks. It's a little bigger looking than a Canon 85mm f1.8, but it weighs about twice as much. Still not as heavy as a 70-200 or something but definitely substantial.