Pros
Direct access to the founding team and hands-on product work.
Cons
I worked for months on the basis of verbal promises — equity, code ownership, a bonus, and a leadership role — that were never put into a contract. Every time I asked, the contract was "almost ready." It never came. The only agreement I ever signed was an unpaid internship contract. My work had real value to the company, but I was never fairly compensated or protected.