I actually just started playing this a couple nights ago from when I picked it up during the last amazon price cut (I think it was $15 or so). It seems like one of the better training games for the DS and I can see it honestly improving your vocabulary in the long run. A great deal at ten bucks.
I have the Wii version, so I can't say I have personal experience with the DS version. However, I can say the Wii version is a good piece of software, and unlike many of the other so-called brain training games, you probably will learn something useful from My Word Coach.
The DS has the advantage of the stylus, which is a much better method of input for this game than the Wii remote.
For $10, if you like word games or really like training games, don't hesitate.