I picked up Burnout Paradise, R&C Tools of Destruction and Persona 3 FES from Gamefly. I knew the disc was in good condition because I was renting it at the time. The box arrived in excellent condition 2 days later with all the manuals and added paperwork inside. The boxes are never handled since only the discs are distributed via the rentals so they are like-new. I've never bought a game from them that I wasn't renting, but I've also never received a broken game either. My experiences are very positive with their for-purchase feature.
" I share the above thoughts. The cases usually haven't been abused and the discs always look decent. Plus you don't get those nasty used game stickers that Gamestop puts on everything. "I hate those. They put them on the insert, not the case, so you usually can't remove them without damaging the insert. I prefer not to buy used from GameStop unless the game is ridiculously cheap, or hard to find elsewhere.
I should add, I did have one problem. When I rented The Wheelman the disc was unplayable. I reported the problem and they sent a replacement, which didn't arrive for two weeks, so I reported that problem and got another replacement, which showed up the same day as the first replacement. However, besides that one issue, I have had no problems with either disc condition or mailing time.
I purchased Dead Space and another game (can't remember which ATM) from them when I was a member. As everyone's saying, the discs are usually in really good condition (and pretty much always functional) as are the game boxes and manuals.
If you're unsure about the quality, rent Bioshock 2 from them and then choose "Keep Game" if you like the quality of the disc they sent you, since that's what you just purchased. Nothing like actually trying the product before you purchase :D