Something to understand with rental cars - the CarFax is complete crap. All the work that has been performed was completed by the rental company and is not reported to CarFax. So don't use "it has a clean CarFax" as a reason to buy a rental. They don't report that a car is in a wreck b/c then it won't sell as well. they have no reason to report it to CarFax.
A good friend of mine is a sales guy at Enterprise, from him: the best rental cars (no wrecks, lower miles, upgrades) are sold by the company. The next tier are usually sold by dealers (high miles, minor damage, standard used car issues). The bottom tier (could be a wreck, tons of miles, no options, older, etc) are sold by used car places since the carfax is clean they can usually sell it for more than it should be worth with a true history report. For Enterprise, what they don't choose to sell, they auction off the rest and that's how they tend to fall out, but sometimes used car places will buy a 2nd tier car if they want to pay for it.
I've bought 2 used rentals in my life and both were good cars. One from a dealer and 1 from a rental company. Just make sure - like you should for all car purchases - that you inspect that car (either yourself if you know how or pay a mechanic to). Also, understand that maintenance will likely be do sooner than normal (30k miles a year rather than the normal 10-15k for regular drivers) so even if the car is only 1 year old, it has 2-3years of it's life taken from it.
Last tip - check the tires. A lot of places will use 40-50k mile tires and sell the car at 30k miles. So the tires might be almost gone already. Just make sure to note that in the price/use it to negotiate.
Benefits: newer model, should have been kept up to date on maintenance
cons: Unknown history, maybe from a different state (so issues like rust from snow people in FL aren't used to looking for)