Pros
Startup life can be thrilling. You've got a mission, a vision of what the company will be, impossible deadlines with too few people trying to balance too many hats while juggling lions, anvils, feathers and banana slugs, yet somehow your small band of warriors bond, laugh a lot and get it done. And there could really be a shiny pot of gold at the end of the tunnel.
Cons
Well, kind of the same list as the pros. Some people want a clear structure to their job, and you're not going to find that here. Every person is expected to do everything that their (future) assistants will do plus their own job, plus the job of the person not yet hired. Then there's the founder. He's old school (which isn't all bad), likes to lecture, gets impatient, doesn't always have time to clarify what level of detail he wants to hear and how much control he wants -- so sometimes you feel damned if you -- and if you don't. And like any startup, there's a chance (well, several chances) it could fail. But of course that's true of established companies too. Or if the company doesn't fail, they can just cut you loose anyway.