I studied Japanese and lived there many years ago and have been trying to keep up with the language every since, with varying success. I've purchased some Japanese games for the DS recently, which have been useful for Kanji and Kana learning (Kanji Sonomama DS Rakubikjiten and Tadashii Kanji Kakitori kun - both available from Amazon Japan).
I think this is the first game released for the DS that takes English speakers through the basics of the Japanese language. English learning games for the DS at all levels are very common in Japan, so this is well overdue. The Japanese games for learning English often don't have enough Japanese in them of the right level to be useful to English speakers, so this is a really interesting development.
Unlike books, this is very interactive and the games are very effective for testing your memory and learning. They allow you to practice and learn very efficiently if you are short of time. I don't think you can learn to speak Japanese fluently just on the strength of this game alone, but as a companion to a good teacher or textbook such as Japanese for College Students: Basic Vol 1 (Japanese for College Students Vol. 1)
, it is very useful. For me, as a revision aid, I have found it very helpful.
The main gripe is, as others have noted, the errors in the stroke order for the Kanji and Kana. You can work round this, but I suggest using a textbook, or Kakitorikun if it is important to you to learn these correctly.
The grammar explanations are complicated, because Japanese grammar is actually quite complicated in places and its difficult to condense into a DS screen. Again, I'd suggest A Dictionary of Basic Japanese Grammar
or equivalent here.
I find the bridge builder game and the flash card game on Audio only (you hear the word spoken in Japanese and choose the English option) to be the most useful so far. The write card game is also useful, but spoiled a little by the dodgy stroke order on a few of the characters. The microphone play-back function where you can compare your pronunciation to the game's voice is also very useful indeed, particularly if you are new to Japanese.
I think the built-in dictionary and phrase book would be very useful too, although I use Kanji Sonomama's dictionary now.
If you already have a DS and are serious about learning Japanese, you should get this game to help along your study, particularly if you are not fortunate enough to have a Japanese speaker giving you regular lessons.
I intend to buy My Chinese Coach when it becomes available in the UK, and I'm also looking forward to the DS Mind Your Language Japanese game, to see how they compare.