Join Date: Jan 2009
Location: Worcestershire, UK
Depends what you are fighting against, how you get the FNP and how much it costs.
I'm assuming that it would come from a special character and that he would be quite expensive. If so then its almost useless- while it would be great against normal bolter fire or equivalent you would only have 1 squad with its effects... which means that you would attract S6+ fire (which would be instant death and no FNP)- almost every army has enough S6 weapons to be able to use them on 1 of your squads
Nids, Marines, Chaos, Tau would have no trouble. Orks would care much (they'll beat you in combat anyway) and eldar could either have the S6 weapons or would just use power weapons against you.
Basically, unless its either cheap and/or available on many different squads at once (meaning the enemy's S6 weapons wont cover it and they'll have to resort to the basic weapons) then its not really worth taking.