The "keep it" feature is nice, though I don't know if I'd want to buy a game that was used as a rental. People don't take care of rental property, it's a proven fact, so who knows what kind of condition it'll be in. Do they send you the case, manual, and inserts too, assuming they haven't gotten lost/stolen?
The main complaint I've heard about GameFly is the transit time, that it can take up to a week to receive your next game. For a service where you pay monthly, that seems pretty dirty floors.
I managed to sell Boom Blox for close to what I paid for it, so it ended up costing me close to what a rental would have anyway. Of course, the selling process is more hassle than returning a rental, but it still allows you to buy with confidence.
I just noticed that you're a new member (despite joining three years ago. Been there myself though

). Welcome to the forum!