I would say the only startup worth working on is your own.

Working at someone else's startup is a recipe for getting exploited - overworked and underpaid. And the worst thing about it is that it's all couched in an atmosphere of guilt trips and taking-it-for-the-team.

Perhaps there are exceptions. I just haven't seen any. I'd rather work for The Man in my day job in a cold corporate environment where everything is explicit from the get-go, get well paid for it and crank out code for my own startup in my free time.