I had a good experience working with awesome developers on a startup, and it's a great opportunity to take on some real-world work responsibilities. You get a small percentage of the startup for your work, which is a cool prospect at first.
Cons
Most startups straight out go under, and the few that survive don't necessarily make it big. The equity that they give you is basically worthless unless the company makes millions. The worst part is that you don't even get to control whether or not you sell your equity. Open water decides when they're selling the equity based on a set value.