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
Cpl Williams, Evan T. USMC, 08FEB2010 ~
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.