At my last full-time job I fought hard for 2% of the YC2010 company and exchanged a disgracefully low salary for it. At some point I got tired of slaving away so that the "founders" of this start-up would become millionaires. That time was when I did the math and realized nothing anybody could do would make this company see more than a $20m exit meant I had a one in ten-thousand chance of seeing $400k after 4 years. So I quit and I expect to gross $200k over the next year while working roughly 1,200 hours.
To work for a start-up is to be a sucker.