With Magic Resistance and multiple foes, I've not seen it 'ruin' a fight. It can terminate a single-monster fight very easily though. Against a quartet of Nalfeshnee it usually gets one, then the caster has to maintain concentration the rest of the fight vs those nasty pig-bites.![]()
I mean, sure, but...4 Nalfeshnee is a deadly encounter for even 4 20th level PCs!* Is that an encounter you've run? Or is it similar to the above posters and you don't use the encounter building rules?
*(Mind you, a 5th level Banishment still has good odds to get rid of two of them: the demon needs to roll a 12 on its save, higher if the caster has any item that raises her spell DC)