Gamefly is great. The turnaround has been very quick for me. Also, the rewards and free shipping towards used games is nice, especially when they a have a decent sale.
If you want to really, really get a new game that is low on availability, the best way to guarantee its arrival is clearing out your queue. As long as the game is actually released, they will send it to you, no matter what. The only issue is that it might take awhile to get there, because they might send it from a distribution center that is not the closest to you. For instance, I really wanted Assassin's Creed, but it had low availability. It got skipped over three times. I cleared my queue, though, and left that as the only title. It was shipped out, but it from California.