Kakitorikunis great for beginners. Its target audience is elementary students in Japan, so the difficulty level isn't too high. The only problem is that it assumes the user knows the meaning of the kanji, so there isn't much instruction on that. It's great for writing and reading, though!
Nazotte Oboeru is my favorite Japanese learning game for the DS so far. It's fantastic! You learn how to read and write characters via an intuitive interface. It's probably best for intermediate to advanced users, but once you get started with it, you can learn a tremendous amount!