I'm going to jump in with Levian here and say that a basis with a good textbook and classroom education is a HUGE advantage. That's not to say you can't learn a language by diving in - as a matter of fact, that really helps too. (Of course, I'm talking about living in Japan here, none of this "I'm learning from manga" crap - that's what I think that is, crap) When I read something with 俺は in every sentence, it hurts me. A) It doesn't need to be repeated, and B) In the absence of "pleasant company" it reads to me as more rude than anything.

I'm saying this because I live in Japan right now and my speaking has improved by leaps and bounds. So has my writing, but that's because I'm still taking classes, with teachers and textbooks. Without that I still wouldn't understand the backgrounds on a good half of the things I am able to say. And I still suck at Japanese.