I got it to see what it was like. It seems fairly decent - the best bit in my opinion is being able to say a phrase into the microphone and seeing how well your pronunciation matches the instructor.
I recommend getting something like the DS headset and microphone, though, as trying to repeat the French into the microphone requires getting up close and personal, which is very awkward.
I haven't played it enough to say if it's a good teacher, but certainly to supplement French lessons it'll be really helpful.