Well, yes, it's just a Nintendo DS. But make sure to buy that hardware that allows you stick a 4 gigabyte mini-SD card in the thing and run whatever you want.
What are some things that this set up can do? Let's see:
Think about all your favorite DS games. Now imagine being able to load a whole bunch of them on a single cartriage and play them for free. I'm not condoning piracy, this is just what you could do if you wanted to. By downloading Brain Age, a Megaman game, and the new Advance Wars, you've more than recuperated the cost of the hardware. Next, think about how that was illegal as hell, and feel guilty about how you just screwed over all the game programmers and their families, you inconsiderate bastard!
Alternatively, you could download a bunch of freeware games instead.
There is a lot of homebrew software out there for teaching academic subjects. One very well-designed piece of software that was shown to me was a simple dictionary. It was smooth and easy to use. For instance, if I wrote down an Asian character on the touch screen, it would look up the definition in Chinese, look it up in Japanese, translate it to English, and pronounce it for you, if you so chose.
Another neat thing it could do was teach you actual subject matter. For instance, say you were trying to learn another language. It would explain the material, quiz you on the material, pronounce things for you, keep track of the things that you got wrong, go back to the things that you missed, and even give encouragement appropriately. The person who was showing me the product told me, in Chinese, "I felt that I was forgetting my Japanese, so I started using this Japanese program that teaches Korean." Yay, fun!
C. Instruct you on Cooking
Pick a recipe; or, search hundreds of recipes based on what ingredients you have available, the number of people you're serving, the amount of time you'll need to prep, etc. The DS will then go through, step by step, exactly what you needed to do for preparation and show you pictures and explanations on how to do it. The pictures are clear, and the product will even read the steps out loud to you. Everything is well ordered, so your carrots are chopped at the beginning and not right before you're hurrying to throw them in. Furthermore, your hands are probably busy, so you can just set the DS on the counter and voice-activate it. Just say, "next step", "repeat that", "louder", etc! Basically, it's just really well designed (if you like Japanese food, anyway).
In general, there are a number of things that I noticed that made the setup sticky. It was easy to use, it was useful, it was easy-to-use enough and useful enough so that you used it, it was hella personalized (the one that was shown to me was completely pink with a pink stylus and pink earphones). Furthermore, you loaded the applications that were personally useful (For instance, I'd probably have the Japanese - Chinese - English dictionary, but not the Korean or French ones). In the end, everything was so smooth with cute graphics and simple animations -- the thing just stuck with me.
Most of the apps are homebrew, so there is a pretty active community. I heard that high-schoolers in Japan would use it to study, because it's useful (and fun!). For those of you wondering about availability, to the best of my knowledge most of the programs are in Japanese, with English being the second most common.
In any case, I want one!