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... ) then remembered what I was supposed to do. Slapped magazine, charged, tapped forward assist... and it felt mushy. Says I to myself: "Huh.. wierd...." The bolt wasn't all the way forward, and I had to hit the FA a few times. Then I settled back into firing position and completed the qualification course.
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.