I bought my 2006 Monte Carlo as a rental with 9400 miles on the clock at the end of this past February. So far I've run it up to 22,500 miles and it's been great; only had one big problem about a week after I got it, the oil pan was replaced (evidently because it was dented) and the tech didn't torque the bolts on the A/C compressor properly, causing it to drag and subsequently snap the belt. Chevy paid for a rental (a Buick LaCrosse, for 5 or 6 days, I don't remember) and reimbursed the hotel night (I was on a trip) and the gas I used to go pick the car up and return the rental.
When I purchased the car I had run a Carfax and knew it was a rental, I only later found out (the service manager gave me a print out of the vehicle details from their internal system) that it was owned by Hertz and operated in San Francisco, California. I saved $10,000 over the cost of a new one and couldn't be more pleased.
As long as the car has not been in any major accident (which to someone with even rudimentary training can immediately spot) and is clean and runs properly, I have no qualms about buying an ex-rental and saving thousands and thousands of dollars. In fact, I'd sooner buy an ex-rental than an off-lease, as with the rental at least someone is taking care of maintenance (the rental company) where as with the lease the owner often does the absolute bare minimum to meet lease requirements.
As for mileage, when renting the Buick, I actually asked the manager about how they determine when to sell off the cars. He said that for them (Enterprise, who is my favorite rental company) it is based on the market value of the car over time versus the amount of time the car is rented. For example a high-end rental (like a Cadillac) may not get rented often, and will lose value quickly, and thus will be taken out of service much sooner than a Cobalt which is rented often, and does not lose value as quickly as the more expensive cars. Thus the less popular, more expensive cars (such as my Monte Carlo, for instance) get taken out of service sooner than the cheaper, more often rented cars (like the Cobalt or whatever).