I used one a lot in Black and would make it disguise as a Ghost type, that way it discouraged Fighting attacks (and typically made them use a Dark move), which would buy a turn for a Nasty Plot, after which he's a decent sweeper, but far from the best.
That trick only really works properly once. After the first fight, all my friends would expect that the Mismagius I sent out might not actually BE one, and it turned into a 50/50 game of chance of them guessing if it was Zoroark or not.
In-game AI doesn't understand Zoroark's ability. If the opponent thinks Psychic is the best move on whatever Zoroark is disguised as, it will spam Psychic even though it's not doing anything. It was pretty easy to abuse on stuff like the battle subway.