[sbisker]

It's worth it if it's a lot of equity. And if you have your lawyer look at the equity contract before you start.

Get your equity contract drawn up before you do a thing. Set up a schedule for your equity to be awarded as your work progresses, not all "at some point in the future" - to ensure that even if you get canned quickly (or they raise money and decide to squeeze you out) you still get something for your trouble.

If they're not willing to give you a very strong equity offer (in writing), try to get them to agree to a deferred salary agreement. Again, in writing, where if they raise money, they're actually going to give you some of it. Only do this if you feel fairly confident they will in fact raise some amount of cash someday (or you like the project enough that you're willing to take the risk.)

If they won't agree to either of these things, run.