I currently do this. The time it takes for them to send me a game is about 2 days because the hub is in PA, vs netflix who's hub is right in Portland which is near me so the netflix takes only 1 day to arrive. Your deliver time is going to depend on where you live vs where the gamefly warehouse is.
You put the games in a queue in the order in which you would like to receive them based on how much you want the game. IE: the first game in your queue is your most wanted game and so on down to 50. You are not guaranteed game number one, as the availability varies. You might get any game from one to 50. They go down your list until they find a match with something that is currently available. When you put the games into your Q, they tell you whether the game is immediately available, or high availability, low availability, or whatever the status is, so you can move the games that you want that have a higher availability up to the top of your Q and you have a better chance of getting those.
I think the service is good if you like to play a lot of games because you can play games you normally would not ever buy. If you want the latest Call of Duty or whatever you might be better off buying it because everyone is going to be wanting to rent that game.