It is good for the purchaser of the game because they get it at a cheaper price and stores like GameStop make tons of cash, but consider the ramifications of supporting the used game industry.
1. Most importantly the publisher and developers of said game make nothing off used game sales, so you are not supporting them to stay in business and make games you enjoy. You are simply lining the pockets of game stores like GameStop, who made I beleive I read 4.4 billion last year and is opening 400 new stores.
2. To make all this cash they ripoff gamers by offering far less than what a game is worth. For instance, you buy a game and decide you don't want to keep it, so you go to GameStop and they offer you $25 trade in, then GameStop turn's around and resell it for $54.99 ($5 cheaper than a new copy during the release timeframe) and they have more than doubled their money.
So the question is do we buy games to support developers or do we buy games to support a retail chain. You make the call.