Wait $100k salary with 6% equity at max sounds not bad at all if you believe in their idea. Although I highly doubt they're a once in a decade opportunity that will be changing the world in a better way.
Edit: To back this up a little more:
To buy a median house in SF you need to make ~$142k.
http://www.washingtonpost.com/blogs/wonkblog/wp/2015/03/08/m...
The 95th percentile in income is WAY higher in SF than anywhere else, and so is the cost of living.
https://twitter.com/conradhackett/status/578004292151341056
My opinion is that if you're not clearly putting me in a comfortable position financially, why would I ever make such a sacrifice?
To put this in context: I'd consider myself average, maybe slightly above in some contexts, below in (many more) others, and my current salary (full-time, all-in) is closer to $200k than $150k. The majority (about 80%) of that is base salary, not "bonus", stocks/options, or benefits. If I can get a ("senior developer" level, not management) job with that compensation, the people in this valley who are much better than me (and that would be most, or at least many, many others) ought to be able to command significantly more.
It is my second job in this area, and I'm a bit (~15%) better compensated this time than last.
If you have the type of resume they're looking for and aren't asking ~$50/hr, you're either not very good, or more likely selling yourself short.