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
NCO of Marines, 3rd Award Expert Rifle, 236 KD Range
D Co, 4th CEB, Engineers UP!! OEF 21JUN-20SEP2011
REV. 19:11 And I saw heaven opened, and behold a white horse; and he that sat upon him was called Faithful and True, and in righteousness he doth judge and make war.