Yeah i've had a few FWB's.
I think I used to prefer having this type of 'arrangement' because I wasn't really ready for a committed relationship, and felt that I had a lot of self-evolving to undergo before I could fully, happily commit and be, in my mind at least, a worthy partner to somebody whom I really wanted to be with for the long run.
FWB's are great if both people involved are totally honest and vocal about their expectations from the start and periodically throughout, and are mutually aligned in their wants/needs from each other.
I suppose it is a bit like having a 'part-time', ad hoc, private, pseudo relationship, with none of the commitments, responsibilities or pressures (predominantly), but conversely, (for most people) no full personal security or emotional and life fulfilment.
They are, generally, like eating fast-food take-out snacks when on-the-go, as opposed to well prepared, favourite home cooked meals.
They have their place, but their expiration date is a limited one (one will outgrow the arrangement before the other/ will move/will find somebody else/grow bored of the limitation/develop more attached feelings than the other et al).
I'm pretty happy with the few I've had, some of them will be guys that I always regard as true friends in the future, but I'm ready to move on to the real deal now, in that sexual-committed-relationship facet.
Well, with the right guy (for me) of course, who, as coincidence would have it, I think I may have very recently met, and surprisingly enough, the attraction seems mutual ;P - but I'm not about to jump-the-gun just yet and *jinx* it...
Still, sometimes something clicks and ignites instantaneously (and occasionally mutually- how rare!), and it's hard to (want to) rationalise against such a lovely, invigorating and rare phenomenon ;]