Gamefly is like netflix. You rent as games you want for as long as you want. Depending on the membership you sign up for. I think its like $9.99 for 1 game at a time and you keep it as long as you want. The price goes up for the packages for more games.
You just put the game in the mail and get a new one. I say its worth it. Especially if your thrifty and like to try games first.