I've used stealth as Batman and Robin in Legends PVP, it's not really helpful, but it's can be neat to use.
Disguise only works outside of arenas and legends. You can surprise players equal or lower than your level as they are questing or whatever. --- "In life, the road to darkness is a journey, not a light switch. - Lex Luthor
IIRC it's just make you harder to detect by players a few levels higher than you, but it can be counter with the iconic passive power, X-ray vision, which makes the users detect players using stealth --- "In life, the road to darkness is a journey, not a light switch. - Lex Luthor
It's useful for solo leveling or if you have friends with patience but I find the slow pace of it a little boring. As for end game, I don't think it would have much use. --- My power is discombobulatingly devastating. I could feel his muscle tissues collapse under my force. It's ludicrous these mortals even attempt to enter my realm
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.