[WingsToDiscovery]

I use it right now in class. It's really hard to go into great detail, but...
Positive: It will teach you all of the grammar/writing/vocab to form a solid command of Japanese.
Negative: It focuses solely on keigo, so everything you say will be super polite and unnatural. You'll have to relearn a lot of material to sound casual.

So basically, you'll be able to understand/speak/read/write Japanese, but you'll be talking like you're at a job interview all the time.