About the Guide to Japanese Grammar: it's a pretty good site for learning the details of grammar, if you've already been learning Japanese from another source. That site also has a structured lesson plan for beginners (
The Complete Guide to Japanese), but it's not very far along at the moment.
My brother tried Rosetta Stone for a bit, and from what I saw, it's not very useful. I imagine it works decently for languages that use the Latin alphabet, because you can read it. However, with Japanese, they just throw you in to looking at pictures with hiragana under them, and you have no idea how to read them.
The "natural" process of learning a language, i.e. seeing objects and learning what they are called, only works for children, because they learn to think in that language. By the time you've already mastered your native tongue, you can't rewire your brain to think in a new language. You just have to learn the new language in terms of the old language by taking lessons taught in your native language.
I just started taking Japanese in college this semester. We use
Elementary Japanese, which is designed for college students with a good grasp of English. It was written by the professors and lecturers here (the greatest public university in the world, I might add). It goes over a bit of the details of grammar, like I mentioned in the first link, which I enjoy learning about. The second year course uses
Tobira, Gateway to Advanced Japanese.