Gamefly is my preferred place to get used games. I've yet to have a single disc come that wasn't mint, to my surprise. I've bought PS3, 360, and Wii games from them. The cases and manuals are mint, too. I expected the cases and manuals to be great since they don't send those out, but the discs being mint has surprised me, particularly the Wii titles.
They usually end up running good sales on titles, so just keep checking back. The problem there is they run out of anything good really quickly, so don't wait around if you see something. If I needed Batman, I'd be all over it at that price. I did happen to be a Gamefly member all but once when I bought from them, so I only had to pay the shipping price the time I wasn't a member.
Specific games I remember buying include:
PS3 - Ninja Gaiden Sigma, Lost, MLB 08 The Show, Uncharted, Call of Juarez 2, Spider-man 3 (GOOD game, unlike it's 360 counterpart), Godfather 2, Resistance
Wii - Super Mario Galaxy, Victorious Boxers, Punch Out, Destroy All Humans
360- Too Human, Call of Juarez 1
I know I've bought more for 360 and PS3, but those are what I remember without looking at them. So yeah, I've had 100% good luck with Gamefly so far and would highly recommend them, but I've seen others say they've had less than mint discs, which doesn't surprise me at all. Maybe my shipping facility doesn't get as many rentals going out, don't know.