No doubt learning Japanese talking to real people is even more effective, but that's not an option for everyone.
This is probably the next best thing: an interactive course. It takes you from the very basics of Japanese to a level where you can understand simple written and spoken Japanese, and if you're good you can even write or speak yourself at the same level. It effectively uses the iPhone/iPod user interface to be more than just a textbook: the strokes of the symbols you learn are drawn in slow-motion, you can hear the pronounciation of almost every Japanese phrase that is shown, and there are quizzes to test your skills. Some useful information about Japanese culture is also included, including hints about when, and when not, to use those phrases you are learning. The author clearly loves the language, has experience teaching it, and knows how to pace the material. I'm willing to foregive him his sometimes corny jokes. :-)
Most definitely worth the money.