I bought an ex rental car, it had its complete service record (importantly, had been in no accidents) and was an excellent price. As a Uni student at the time, price was important because I didn't want a car loan, but safety was just as important. It turned out to be mechanical reliable with no expensive surprises.
I wouldn't be put off a car because it was ex rental, just get it checked thoroughly before you decide so that if any major repairs are imminent they get factored into the purchase price you agree to.