I'll let Dan answer for himself, but for every person that pushes their rental cars to the limit, there are 10 more who baby them because they're nervous about getting screwed when they give it back. Either way, rental cars are better maintained throughout their tenure as rentals than privately owned cars (there are individuals who take phenomenal care of their cars but it's rare). It's still a used car, you have to get it looked at before you buy it to make sure it's in good shape.
I have yet to find a person who bought a rental car and regretted it, while I know of many that are extremely happy with their purchases. Most of them don't buy directly from the rental companies though, they get them from a guy who buys them at auction. It could be that's a safer bet.