Bottomline is, always protect the downside. Startups do offer a shot at becoming rich, but they are more likely to fail than not, so in the event of the business going down you still want to show something for the years spent.

Basically: make sure to get a fair salary down the road, or at least make sure you learn something that you otherwise couldn't (and makes it easier to find a better paying job in the worst-worst case scenario).