One other comment I'd add to the other two posts (which are spot on) is make sure the terms of the equity are clearly understood. Consider both the type you're getting relative to the rest of the cap table (typically it's going to be standard founders/non-protected junk equity) and of course the valuation of the company as a whole. Unlike capital investors there won't be much you can really argue from a valuation standpoint but knowing what (and importantly how) they define as the companies total value will help you understand where your money will be going next and how likely they are to succeed at progressing (30% of 0 is still 0 after all.)