It is OK. It can give a lot of pokes a free retreat, but so can a lot of other cards, and people don't play them. It doesn't combo well with empoleon because Empoleon usually plays Cessation Crystal, thus shutting down Mantine's pokebody. It would take up two spaces in the deck to work. One benefit is that you can use mentor to grab the full line. It is really nasty how it can only attack once every-other turn. Good resistance, Weakness wont matter a whole lot, and mediocre HP. I don't see what the rave is about.
5/10
EDIT: ^ That score is for modified