Besides the military M16 haters who has ever used the forward assist on a AR type rifle?
I have used it to effect exactly once. I remember it quite clearly... qualification day in basic training.
I was firing my second magazine, from the prone, and a few shots into the sequence, nothing happened when I pulled the trigger. I looked at the rifle like it was stupid for a second (such was not actually the case...

My guess is that the cartridge was deformed in some way, since all the others fed fine. I've thought about that incident a few times since, too, along the lines of it might've been better for me to just eject that round and chamber the next one, which chambered fine. Thing is, I would have been two rounds short, instead of just one--and I only hit 23 targets that day... the bare minimum for qualification. So... I was glad that FA is there.
On the other hand, the FA is also an integrated part of our immediate action drill--mainly because it's there. So, I've actually used it many times... but only once that it mattered.