You guys are brave. Flying another persons plane seems rather crazy to me. But to the newbie (me) you instantly earn their respect when you bring it down in one piece.
That's why they pay me the big bucks. Oh, wait, I don't get paid. Nevermind.
Actually, it's a lot of fun. And a challenge all it's own. And I get to "sample" lots of different planes, radios, and engines.
Of course, most of the time the plane being flown is a trainer, and these days, with the quality and popularity of ARFs, 9 times out of 10 the test flight is totally uneventful, almost boring. Well, never boring. But that 1 out of 10 sure makes for some fun and excietment. I don't know about other guys, but I get a huge charge out of bringing back a funky flying plane in good shape, when I know a lot of guys would have reduced it to splinters.
Back when most planes were kits, first flights were always more exciting. And yes, I do seriously go over a new plane to minimize anything that could go wrong.