The program will run you $60 for six months. You can drop in to your local GameStop to get a different game at any time, but you can only have one title out at a time. At the end of the six months, you can choose one game to keep for no extra cost. You'll need to be a free or paid member of GameStop's Power-Up Rewards program to sign up for the rental program. Whether GameStop wants to monetize its huge stock of used games, as Mashable's Adam Rosenberg notes, or just wants to get more people into the store, PowerPass could likely be a hit among gamers looking to try their games out before buying them.
Update: Shortly after publication, GameStop confirmed to Engadget the details of the PowerPass program. This post has been updated to reflect that confirmation.