By my experience (I've gotten job offers from a few startups), most startups (especially after their series A) offer employees (at least engineers) a miniscule amount of equity (a small fraction of a percent). Even if these companies would have great exits (tens of millions), their engineers would hardly get enough for a down payment on a house in the Bay Area.

Summary: don't expect the equity you get from working at a startup (unless you're a VP or an executive) to be worth much. Google-like events where many employees got rich are few and far between. If you do work for an existing company, I suggest looking at salary more than equity as a means of making money. The difference between making 120k vs 80k over 4 years is 160k -- more than you're likely to get from options, and less risky.