In short.. awesome first half, standard (i.e. mostly boring and formulaic) romantic comedy for the rest of the movie.
It had some awesome scenes in the beginning, especially the raunchy sex scenes with moments everyone who's got some experience in that area will recognize and laugh about. Sadly this funniness instantly evaporates when they inevitably fall for each other (come on.. that's not a spoiler.. it's a romantic comedy!
) and they go through the motions before ending up with each other.
It's much like The Proposal with Sandra Bullock and Ryan Reynolds.. Sandra Bullock is brilliant as the bitch boss and Reynolds as the assistant who hates her with a passion but once they start seeing each other as persons with qualities the movie turns dull.
FwB is ok for a video rental Sunday afternoon when you've exhausted your DVD rack and are not in the mood for some violence or other stuff.