I've had my credit card not accept my Gamefly payment because it put me over the limit on the card. They send an email telling you that the card was declined but they will continue to try charging it. I always paid the card enough money to accept the Gamefly bill within a few days and everything became all good again and I never sent a game back....but I'm sure if you don't pay your bill for long enough they'll want the games back. Like Sergeant Pope asked, Why would they let you keep the games for free?
Usually my games arrive 2-4 days after Gamefly has shipped it. However, I have rented Zelda: The Twilight Princess for the wii 2 different times; both times it took over 8 days to receive after being marked "shipped' by Gamefly.