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As SwiftStar says, if you have a mental controller, then disguise is surprisingly useful for leveling. While it limits your movement, it has a very powerful stun spell (sleep) and a one-shot crit hit that wipes out 2/3 or more of most NPCs health bar before exposing your normal self. Disguises come with their own load bars, but only the sleep power can be used without dropping you out of your disguise. I've used it even up to the level of doing Gorilla Island alert, and the stun spell still works.
It probably diminishes in usefuleness after you get to 30, but since it is in the branch leading to other controller powers, it's worthwhile getting.