The Nalfeshnee is a high-ranking demon, somewhere between the Glabrezu and Marilith in the abyssal hierarchy.
Nalfeshnees have the standard things one would expect of a demon: natural armor, high stats, energy immunities, SR, DR, telepathy and energy resistances. They're Huge with 15 feet reach, making them good for meleeers. Nalfeshnees also have two claw attacks and a bite attack, but the claw damage is only 1d8 and I'd recommend just grabbing a greatsword.
The demons possess several special abilities. Most iconic is the confusingly-named 'Smite'. It's a delayed daze effect that doesn't affect demons. Interesting, and the strange name allows feats like Sapphire Smite to boost it.
The spell-likes are not too great, but there's a few good ones. Call Lightning deals pitiful damage, even when used in a stormy area (which you have no way of generating). Feeblemind is a death sentence for arcane casters, but it's been available for quite a while now and you won't need it too often. Greater Dispel Magic will of course always be useful. Slow is a good debuff, but again far too low-level. Greater Teleport is par for the course on a demon, and Unholy Aura remains a decent party-wide buff.
Finally, nalfeshness can summon vrocks, hezrous, a glabrezu or another nalfeshnee: let's just say there's worse things to summon.
All in all, nalfeshnees are not bad, but they lack the strong, unique abilities other demons seem to possess. They seem about equal in power to a glabrezu to me: while they can summon the latter, they can only do so for an hour per day at most. +1 LA for now.