I've never used Rosetta Stone before, so I can't say much on it. Personally, if I want to learn a new language, I'll get a tutor or a native speaker that's willing. That way, you can have 'practise conversations' with them and have a 'feel' of the language. I'm not sure what Rosetta Stone does to help with your vocabulary, grammar skills, and conversation skills :/
But that's just my personal preference - I've never learnt a language by myself so don't take my word up for it.
In other words, you no nothing but are happy to share your speculation

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Having actually used RS a little (in my on again-off again attempt to learn Russian, as it happens), I can say it does provide a style of learning which you can't find elsewhere (and certainly not for free), but does benefit from having a good dictionary and other supplementation as necessary. Communication with native speakers goes without saying, of course.