We share our dufour arpege with some friends of ours and it works great for us. It is legally my boat, but we treat one another as equal partners and share the annual expenses evenly. Capital expenses are treated on a case by case basis, but since I own the boat they are generally mine to bear.
Since my current life situation doesn't allow me on the water as often as I would like, I've found boat sharing to be not only a big money saver but also nice to have the boat used a bit more and thus also better maintained. We don't always sail together, but try to invite one another along when possible so have never really had as usage conflict. We don't have any kind of formal agreement, just a handshake deal that is good for one season at a time and an agreement to keep it going so long add it still works for both of us. That might not work for everyone, but we are friends and coworkers and trust one another enough to make it work.
So far so good.