While not a mustang, my other car (2012 VW Golf) was an ex-rental. I bought it for about half the price of a new car and had about 40k on the clock. I got it with CPO and an extended warranty until 120k. It had one scratch on it and was in near mint condition. It was like I practically got a new car for half the price. Now with the mustangs, people could rent them out just to burn rubber and therefore is a type of car that is likely to get abused more often.
If it is not CPO, I would have a dealer or mechanic inspect it. I bought a used mustang and took it to the dealers and paid $80 to inspect it (or you can take it to a local mechanic for cheaper but I wanted ford) and everything was mint. I saved almost half of the price vs new car. Try to get the service records of the car, rentals usually take good care of their cars.
I would say go for it if everything checks out with the mechanic! Edit to give you some price figures (Canadian):
2012 Volkswagen Golf 2.5 Base with Heated Seats- 40k odo (CPO w/ Extended Warranty)- MSRP: 25-30k, Got it for: 16k
2011 Ford Mustang Premium (options in sig)-58k odo (Non CPO/Factory Warranty)- MSRP: 30-34k, Got it for: 18k