I've been out of the game(US armed forces) for a long, long time(22 years or so), so I'm not Mr. CQB, I would think a M4 or AR/M-16 style foregrip would have problems with shooting from the prone or moving about in tight spaces.
The newer milspec bi-pod fore-grip is clever but I do not see much use for it.
The Mag-Pul system makes more sense to me.
I will however state that the green-laser aimer/white light(with strobe) could be useful to a home/property owner or armed professional. Crimsontrace; www.crimsontrace.com
markets a slick fore-grip & the SIG Sauer Stop-Lite is cool.
There are a few practical DVDs & guides for CQB with M4s/patrol rifles.
See; www.uscav.com www.paladin-press.com www.deltapress.com www.gunvideo.com