While the 40K Interwebs are gearing up to cry about 2-wound GK Terminators with Feel No Pain, I thought that I'd bring some clarity to the discussion. What is the real benefit of FNP rolls for different troops?
The math is pretty simple. Whatever your chance is of failing a save, FNP cuts it in half. Thus as the base armor save increases, the benefits of FNP decrease. Here's the odds of passing a lasgun save for the different armor values with and without Feel No Pain.
6+ Armor = 17%
6+ Armor and FNP = 59% (42% Gain)
5+ Armor = 33%
5+ Armor and FNP = 66% (33% Gain)
4+ Armor = 50%
4+ Armor and FNP = 75% (25% Gain)
3+ Armor = 66%
3+ Armor and FNP = 83% (17% Gain)
2+ Armor = 83%
2+ Armor and FNP = 92% (9% Gain)
Yep. If you have a 2+ save, then FNP only adds 9% to your ability to survive a lasgun or bolter wound. While you gain nothing in terms of surviving a plasma or melta wound. So is FNP really that big of a deal? Frankly, if Sanguinary Priests didn't also come with the ability to grant Furious Charge to my BA Sanguinary Guard units, I'd never bother with them.
Dark Eldar, on the other hand, benefit extremely well from Feel No Pain. Give them a pain token and they can get a 4+ save versus Heavy Flamers and Bolters. Not bad at all really. Orks wish they had that kind of durability. It means that once they get warmed up, Wyches can run out into the middle of the field without worrying about any gun under S6.
So keep that in mind before you cry. :)