I've seen this in my own life and in many colleagues, friends, and people I've hired. It's not a guaranteed paycheck, but in the bay area getting a healthy equity grant is part of the standard comp package. Without luck, it can be a nice bonus. With some luck it can really move the needle. And with ISO's especially you can choose when to pay taxes on the income.
I know that outside the bay area comp packages are structured differently -- and often less lucratively.