I'm half of a casual, very informal partnership with our neighbors that has gone on successfully for ten years. It's a little different from what most are referring to, though. Our motivation was mainly in that being two very busy families with professions, businesses, and kids, we had limited time for using a boat- but we live almost on the water. A partnership was a way to have a boat available, without feeling that it would own us. Our boat is an older 33' Scandinavian racing design and is used mostly as a day-sailer. It doesn't represent a very large financial commitment for either of us, so the pressure is minimal. We refer to it as our "neighborhood boat", and it gets used as well now by our somewhat grown kids. We frequently sail together, my partner owns the slip (a financial commitment), I do probably more maintenance and repairs.
A relaxed attitude helps, and as someone said, valuing the partnership more than the boat. Our motivation wasn't financial, but specifically that sharing the boat increased its value for us.