I used to work for Hertz car rental back in the late 80s, early 90s. My brother bought a just off the line 91 or 92 Ford Tempo, the updated "Mini-Taurus" body style. To this day, he talks about that car and wishes he still had it. Just remember, you are buying a used car.
You are buying a used car that no one specific person owned or drove. Some people drive rentals as if they are their own, others just beat the living h3ll out of them. Pick the best one of the lot and you will be happy. Pick the worst one and you may have a money pit. No different than that used car you look at on any other dealer lot. The only difference here is you know it was a rental where in the traditional used car market, you simply have no clue of what it was put through.
Be critical, inspect everything, ask for repair records, talk to the rental/service staff if you can (not the sales staff) because they know fleet numbers and the yard guys know which cars are ragged on because they see them come off rental all day long and you get to know your fleet numbers. Plus, they drive them back and forth themselves, well, in smaller stations anyway. Oh, that's another tip, try to get one that is from a smaller market, not a huge one like LAX, JFK, etc. The wear and tear on the smaller market cars will be less than those turned around and rented within minutes at larger markets. More time to care for them.
Just my thoughts as a former rental car employee and the person experience of my brother buying his Tempo. And I will agree, I made sure he got a good one, the benefit of the yard guy.