Britney - Shows literally no emotion or connection with the acts or show, like she doesn't really wanna be there. Needs to seriously expand her vocabulary and start to give the contestants construvtive criticism. Pretty good mentor though, she seems to know which direction to take her acts in.
Demi - Great judge but awful mentor. She ruined her acts to an extent (although built back credibilty for herself in time with a few changes she made), but overall she didn't know what to do with them it seemed, or let them be themselves fully. Usually gives some very good comments though and says it how it is, often what we're all thinking aswell.