Pre-Order to Try for 48 Hours
When you buy Call of Duty: Modern Warfare at GameStop, you can try the game for 48 hours without any risk. If you don't like the game, you can return it to GameStop within 48 hours for full purchase price in trade-in credit.
This is a Guaranteed to Love It promise from GameStop that is only good for 48 hours from launch, not from the time you purchase. So to get the full advantage of this promise, you need to get the game at launch and then see if you like it.
If you decide that you don't like Modern Warfare, you can take it back to GameStop within 48 hours and they will give you store credit. This is a nice option if you missed the beta or simply want to try the game out.