I bought the game this morning!
I had been waiting for it for so long, so I'm glad I was finally able to pick it up.
From the short amount of time I've been able to spend with it so far (i.e. 45 minutes or so), I think it has great potential as a Japanese learning tool.
After you start the "game", you create a profile (out of 3 available slots) and take a pregame assessment test. You are asked to identify a variety of words based on various translations - i.e. English to Japanese, vice-versa, listening comprehension, etc. After the assessment timer is up, or you get 2 words wrong in a row, you then begin the game based upon the level you tested at. This prevents more experienced students from having to trudge through previously learned subjects.
From there you go on to "Lessons". These lessons are themed ("Months", "Days", "Colors", "Kana", etc.) and you use a variety of tools and games to "Master" the vocabulary or kana.
The games are normally pretty fun and the tools are great. The tools include the ability to compare your recorded speaking to the game's speaker's and the ability to practice writing by comparing your writing to the "proper" stroke order and form.
As you "Master" vocabulary you unlock more lessons and the pattern continues throughout the game. I believe that there are over 10,000 vocabulary to learn and possibly the kanji as well (I haven't gotten as far as kanji yet, but I know it is taught. To what degree it is taught, I do no yet know.)
Separate from the game portion of the title is a "Reference" section. This includes a dictionary, phrasebook and a sketchpad. All useful and some utilize the speech and writing tools in addition to their obvious functions.
Like I mentioned, I've only spent a short while with the game so far, but I'm already pleasantly surprised and I am looking forward to seeing just how in depth My Japanese Coach can take a student into the Japanese language. I'm not sure if conversational tools (like those found here, on JPod101) can be replaced by the game, but it certainly seems like a must-buy for DS owners who are studying Japanese.
がんばれ!