Very interesting responses here I must say.
I am now into the ninth year of my partnership and it has been an excellent experience. All partners have been on the same page regarding maintenance, that is, we never skimp. If one of us brings up an issue, we don't argue among ourselves, we simply get it taken care of. The urgency obviously is dictated by the seriousness of the squawk. But most importantly, we have all become good friends. Whenever one of us needs to do some air work or approaches, we never hesitate to accommodate the request, after all, we get to fly with someone just as passionate about the plane.
There are three of us in the partnership and I would not recommend more than that. I have friends whose partnerships contain four or more and while it does keep the expenses down, I see them encounter scheduling conflicts all the time and that creates friction. In the nine years that I've been associated with my partners, I can't recall one scheduling conflict, although we each know to get the trip on the calendar as soon as the date is known. We don't frown upon someone taking the plane for an extended trip, if someone wants to take her for two or three weeks, that's just fine with us.
Bruce is correct when it comes to selling and finding a new partner. One of ours decided to sell his late last year and it took four months for us to find a new one, but he was worth the wait. His personality and mission fit in perfectly with us and as an American Airlines captain, he surely knows his stuff. If it took us four months in a very densely populated area to find one, I imagine it could take a year or more to find one in a rural area - if at all.
We bill wet because it obligates the person returning to make sure there is fuel in the tanks for the next guy. Although we don't have any formally written requirements, we know to top off if we are returning with a little more than half tanks.
The original poster is correct about interviewing the prospective new partner because the fit is so critical. In our case the three partners and spouses had dinner with the new prospect and wife and we all hit it off very well. Now several months into it, he has become one of us and puts the Mooney to good use. And as far as bringing on an additional partner, our agreement states that the vote must be unanimous as this would keep the peace. Upgrades are always on our mind and again we are on the same page. Over the years we installed a GTN 750, took care of our ADS-B requirements early on in 2015 by installing the GDL 88 and undertook a complete glass replacement project (windows, not panel) and none of us even flinched.
There are some out there who would never consider taking on partners, but I would never consider owning a plane without them.