I joined a pre-IPO company as a contract to hire, and then decided to not join as full time. But I had buddies who joined full time. They all left the company 8 months later after the IPO: the stock price fizzled out.

Just compare your salary and options, with what Google, Apple offer. And also check whether the start up is type A (Uber, Dropbox, etc), type B (box, etc) or type C (unknown potential).