I was in a partnership years ago, which started out pretty OK--compatible people (there were 3 of us), the right airplane (a low time 182), affordable for me. We sat down and talked for a couple hours about what each of us wanted in a partnership, and then put it into a pretty usable partnership agreement.
Over the first few years, things worked really well. We upgraded the 182 a little by improving the avionics, adding wingtip strobes, etc. We agreed that generally speaking, business flying came first, but otherwise, use was on a first come/first served basis, just call the other guys if any of us wanted to take the airplane for awhile. We had an airplane bank account, and the hourly costs we charged ourselves went in there and were used for all maintenance expenses including reserves for overhaul; we paid the fixed costs of the promissory note, hangar, insurance, etc. monthly, each paying 1/3.
But over time, one partner sold his share, then when that second one wanted to move away, he couldn't find a buyer, so the 2 of us remaining bought him out. Things continued to work OK, until my pard wanted a faster airplane--just about the time Cessna brought out the 182RG. While we muddled around trying to decide on buying that, they turbo'd it, and that's what we bought. But I couldn't afford to go half of a new airplane, so we ended up in an unequal partnership, 1/6--5/6 for purposes of the promissory note, everything else 1/2--1/2, and of course, the hourly costs went up exponentially. As you can imagine, he began to act like he was the owner and that I was just a tenant--but he wanted me to handle all the non-flying details like maintaining the charts and arranging for mechanical work. And even if we'd talked about it well in advance, if I took the airplane for a couple weeks such as for vacation, I took a lot of flack when I returned for making the airplane unavailable for him.
It got worse when only a few years later, he wanted to swap the TR182 for a T210. I agreed to a 1/12--11/12 ownership arrangement for purposes of the promissory note, but that didn't last very long--I couldn't afford the hourly costs, and it took all the fun out of flying, not to mention the "it's mine, you're just a tenant" mentality I was dealing with. When I returned from a vacation and took more than the usual flack, both for being gone and for a prop ding, that was the last straw, and I got out--just gave him my share.
So then I went many years of just renting, until I came into an inheritance which allowed me to buy my LRB, a 1963 P172D. I've had her now for 9+ years, and having her all to myself has been great (except when it comes to expenses). I do with her what I want, when I want, whether it's going somewhere or adding to or modifying her. I don't have to consult with anyone about whether something should be done (other than my IA, of course), and consequently, as the gadget freak that I am, she has additions and mods that others might not like but which suit me--the list takes a full page typewritten single-spaced.
The obvious downside of sole ownership is that there's no one to share expenses with, and occasionally that is painful. "Clean" annuals are uncommon--there's always something that needs repairing or replacing, and if the annual itself is pretty clean, invariably I'll want to do an upgrade of some sort. But she's my baby, so I don't too often complain about the expense.
Those are my rambling thoughts, for whatever it's worth.
Cary