These are the pros and cons to me, but I'm not a member..
Pros: Keep as long as you want, no reason to buy games.
Cons: You have to wait a few days for a new game, newer release games may not be available at release
It seems like people only play new release games for a month or two anyway, so gamefly is a great alternative to buying games.
Plus, if you DO decide to get a game at release, you can do the "Keep it now" option and keep a new release game at a discounted price, usually around $45.