Some interesting vocabulary comes up in songs that you might not learn otherwise.
I've just learned
遥かな = far away, distant, remote
and
囲炉裏の端で = by the fire/hearth side
I haven't learned anything but vocabulary from the few songs I've looked at so far and that is always dependent on being able to find the lyrics written somewhere.
Thanks for the comments and agree that doing the transcription is not a easy task.
This is how I do it - I usually listen to the song without looking at the lyrics while I am transcripting for the first time. And after I completed, I look up the lyrics from the net and check my error and the patterns I don't understand, I do learn from those mistakes and error.
I agree with >1 in that learning by music makes grammar difficult, but I disagree wholeheartedly in that one should wait until they're at an advanced level before trying to learn through music.
I listened to Japanese music from when I was a novice - in the beginning, not only does it help you learn vocab (especially if you listen along while reading lyrics and/or translations), but it helps you LEARN TO LISTEN, which is a very important part of the language. Differentiating when words start and end especially when you have mile-a-minute speakers doing their best 関西弁 is an important skill.
Now that I'm at an intermediate/advanced level (I can pass the JLPT3 and I've started the JLPT2-equivalent level at my language school) I'm starting to pick up/recognize the grammar from a lot of Japanese music I listen to. I'm also doing transcriptions and translations here and there to brush up.
Really, the important thing when learning Japanese is that you must immerse yourself in the language - music is an essential part of that.
i agree as well. learning the language is one thing. but listening to it and speaking is another important aspect. this way you dont constantly have to ask to repeat or slow down.
Listening to music is a good way to supplement language learning, but I would never consider it a substitute fo some kind of lessons. I'd hate for my students to learn English through listening to Fifty Cent or Eminem.