Gamefly is great if you get the 2 out at a time plan. This makes shipping times a non-issue, for me at least. It's easily my favorite place to buy used games. Not only do they have some great prices, on some games at least, the games look mint.
I've bought a lot of used games from Gamefly. Every single game came with mint condition case, manual, artwork (no stickers all over everything), and most of the discs have been 100% scratch free. Even the couple of discs with scratches were faint scratches that didn't affect the games at all.
It is definitely harder to get the very latest titles, but if you're willing to set yourself back 3-4 months, there's rarely a problem.
Yeah, if you're wanting the very latest games, just released within the last 30-45 days, I'd not suggest Gamefly for that. It is definitely better for slightly older games and good but short games that you don't want to buy, such as Wolverine, Heavenly Sword, or Alone in the Dark (the PS3 version).
I'd suggest signing up with TheECA.com free for a year, then use their 30 day free trial with Gamefly, go with the 2 at a time plan. 2 at a time is better because that way you always have one game to work on while you wait on your last game to ship back. Just cancel the 30 day thing before the 30 days are up if you don't like it (set up an alert in your cellphone or something) and you're good.