Joel gives a better answer than I do; note that his plan gives 4 developers 2.5% of the company each, but 1%-5% seems like a typical range. The commenter says they were only able to improve their salary by ~20% by taking a BigCo job, so the rest of comp doesn't seem to be too far off the market.
I don't think it's an amazing offer, but then, amazing offers can be red flags too.
http://answers.onstartups.com/questions/6949/forming-a-new-s...