The Marines teach us to use it but IMHO it's fairly useless unless your weapon is dirty and you absolutely HAVE to chamber a round that doesn't want to go in. You tap it and it nudges the bolt forward on a stubborn round.
If you're having to use the forward assist a lot you need to either A) Use better ammo B) clean your rifle or C) buy better mags.
Just my .02
Sgt. of Marines, 5th Award Expert Rifle, 237/250
1st Award Expert Pistol, 382/400
D Co, 4th CEB, Engineers UP!! OEF 21JUN-20SEP2011