Some people swear by them although I never used them much. The key is to position them properly so whatever action they imitate is not wasted.

For example, if you have a character attack a foe and the next turn goes to a Mime, he'll swing at whatever he's facing. If there's nothing in front of him, he'll swipe at empty air.

So you'll have to be good at geographical logistics to get much use out of them although they can be quite powerful. I've even heard of a legendary Ramza + 4 Mime party.