I have found an as cheap and more reliable way is to buy the games cheap and then flog them on. I have a PS3 cus that is better than the XBox

(<-- srry, couldn't resist) and if you browse these forums and keep looking in shops you can pick games between £10 & £20. Sure you won't get the latest games but you hardly ever with rental services. If you trade-in a game or swap it with a mate, or if you have no offline friends (like us gamers often don't) then start a games swapping club at work or something.
I must admit that I am no longer a fan of rental services so I am slightly bias. But spending £15 a month on rental then maybe only geting a game or two which you don't complete doesn't compare to buying the game for £20 and having a good run at it before you are 100% you are sick of it to then get £10+ trade-in or swap for something you can do the same on.
My 2p anyway