How have they worked out with you? I've found that a large portion of people don't actually follow through with what they say. Reliable people are out there, but hard to find. There is also something about working with people in a partnership situation that makes them less reliable than if you have a payee/payer relationship -- even if the payoff is potentially big.
Things that I think help:
Clear division of skill sets/labor
Short term goals, quick turn around times
Someone taking authority/lead even if its a profit sharing venture
