Good advice above!

I partnered up with a distant friend to buy our Marstrøm Tornado, and we are still friends. It has worked out very well for us, but we have needed to work on it underways.
I would not hesitate to partner up with somebody I knew relatively well to buy a boat for a campaign or racing. But for a recreational boat to be used with your respective families, I would demand a clear agreement on all financial matters and boat work. Like what is said above.

After having co-owned our current boat for the last four years with Frode (look at the "first mate" thread, and you get the idea), while hearing and seeing how other boats handle boatwork+financial stuff, I find it very advantageous to co-own the boat. I have a regular crew, motivated to both work on our sailing and the boat. If we want to buy something expensive for the boat, we can split the cost. Since we know each other very well after a while, I have a great friend etc. Driving 15-20 hours to a regatta is much more fun with a good friend. So there are lots of advantages to co-owning a boat with the right person. Co-owning with the wrong person on the other hand, or when the project goes sour.. Ouch!