I've acquired a whole lot of mix CDs over the years, from friends, acquaintances, and correspondents, and I've added commercial CDs to my collection after finding them in the stereos of cars I've rented. (This, for instance, is why I own a copy of the Love and Basketball soundtrack.)
But last Wednesday was the first time I've added a mix CD to my collection after finding it in a rental car stereo. And I have to say, it was pretty disappointing. I liked only about a sixth of what's on it. In the future when I have the choice, I'll think I'll stick to mix CDs from sources I trust.
[Ed. note (2025): This is still not entirely my jam, but I'm also not quite the music snob now that I used to be.]