The Nintendo DS clamshell design means it is pretty sturdy. Still, it's great to have a case to carry it and all its games around in! I got the "Official Nintendo" DS Carry Case NDS-15 to try it out.
The sturdy fabric is pretty waterproof, and the area that the DS slides into is padded and well protected. The pocket on top is great for random stuff - earphones, battery charger, etc.
The front flap folds down to show nine see-through pockets to store your DS games so they stay clean and functional. There's a lower mech pocket to stick old GameBoy or GBA games. The inner side of the flap is a full mesh pocket to hold extra stylusses and other small gaming supplies.
There's a pair of straps that come with the unit. One lets you carry the case easily by hand - while the other is a longer strap for hanging over your shoulder. The longer strap has an adjustable slider so that you can make it longer or shorter as you need.
Highly recommended as an easy way to keep all of your DS stuff together, and keep it safe from harm. While the DS unit itself is really solid and keeps itself rather protected, the games are so small and slim that they're easily squished or lost. With the carrying case, that is far less likely to happen!