I try to take advantage of every preowned offer that comes my way, be it through Gamestop or any other store that's willing to shave a few bucks off its price. I mostly get good deals, and I won't buy a game unless it's complete and in good condition. The boxes might not be shiny new, but hey, you have to find ways to save money in this economy.
Sometimes, however, things don't work out. Yesterday, I found a used copy of Lego Indiana Jones 2 for the Xbox 360. It was only $15 after discount, so I picked it up. After three "unreadable disc" errors, I packed it back up for a return trip to the store. It was the only used copy they had, so I'll just pay the extra $5 and buy it new. Two trips to the store for a $20 game that's everywhere at that price. *Sigh*