I do not mind flying someone elses plane at all. However, there are a few rules that I play by.
1. If they ask me to fly their plane but do not directly assume responsibility, I tell the person if you want me to fly it, I am not responsible for what happens up front. It's not being rude but there is a lot of money involved so if they are wanting me to fly it, its on their nickel. I just want to be upfront. They never get offended.[8D]
2. I know my limits and I stay WELL within them.
3. If everything on the ground does not indicate that the plane is in PERFECT flying condition I will not fly it.

So often I want a friend to enjoy flying my plane and I want to put their mind at ease so they can relax and fly.
Here is how I handle it when I am handing MY transmitter to someone else. 1 rule, 1 statement.
I say as I hand the transmitter; "Here buddy, I want you to fly my plane. Take it and have a ball. No matter what happens I will not hold you responsible. If you crash its most likely its going to be due to equipment failure or just bad luck. Either way I am willing to assume all responsibilty so just enjoy yourself."
When I do this, I do not care if they intentionally point it nose down and fly it into the ground, I said what I said and I will stick by it no matter what. No caviots.

You might think I am crazy, but its my plane and my risk to assume. This is not only something I do, but something that has been said to me many, many, many, MANY times. I would never do this with someone if I did not have a very high level of confidence that they would bring my plane home safely. I have never crashed anyone else's plane, and I have never had mine crashed. The only time I ever saw a problem was about 4 years ago, two friends were switching off with an LT40. On one flight the guy who did not own the plane was doing a rolling circle and the wing came off in flight. The plane went straight in and shattered into 1000 pieces. The owner reaffirmed that he was more than willing to take the risk and re assured the other guy that it was ok. It turned out the the wingbolts block was not secured in and had pulled out. No big deal, it was going to happen eventually regardless.

Those are the rules I play by in this situation and they have not failed me yet.