I have (regrettably) had a friend "with benefits."
In theory, it should be a good idea. After all, we are young, healthy and full of... energy. If you are JUST friends, FWB is a way to have a little "fun" and then be all buddy-buddy after. The best part is you don't have to feel self-conscious about your body, you can be honest about what you want and in some instances, you can just order a pizza and beer and chill out afterwards.
While I was pro-FWB once upon a time, I found two factors that will turn the too-good-to-be-true scenario into a socially awkward nightmare.
1) If one friend is in love with the other and the feelings are not reciprocated
2) If #1 is true AND your friend also happens to be your roommate
Moral of the story:
JUST Friends with Benefits = fun, but risky
Non-recipricated love between Friends/Roomies with Benefits = a VERY bad idea
Learn from my mistakes, fellow FWBers! Cheers!