I have used AFL a few times casually in some personal projects, and it has always performed quite well for me. Of course, there are a lot of weird cornercases which would not occur on real-world (non-adversarial) inputs, but it also found some very real bugs.

(For example, I once wrote a hash table implementation where the insertion and resizing procedures had slightly different views on wraparound, causing failures on very specific inputs. Another time, I wrote some code to buffer out-of-order messages, which would only occur due to a race condition. It was wrong. Both times I had thought carefully about the code, and the bugs would have been painful to discover otherwise.)