I own two copies. Honestly it is tough to say if they are worth it. I think it depends on your learning style. I know they promote "immersion" but I can tell you this:
I bought the Rosetta Stone for Korean before I every came to the country. After having lived there and gotten accustomed to the language I would go back and review Rosetta stone and find out that, although it was correct, it was not practical in everyday Korean. Though to be fair, Korean has different levels of formality and the CD emphasized the neutral, yet formal/polilte tense. Though everyone would understand you if you spoke like the CD, you could only understand others if you were watching the news or at a business meeting. Not really applicable if you were buying groceries or chatting with friends. In fact, you would probably get laughs if you used it with a somewhat close friend or peer. However, after speaking it much better (after 1-2 years) it turned into a nice review of more formalized Korea. I guess you have to keep the language your are studying in mind before you buy. That said, it isn't totally without value. It is accurate, and if you stick with it you will learn a lot of vocabulary (especially nouns and verbs - adjectives were trickier). I guess, I've just learned that not just immersion, but in person immersion is still where it is at for me.
Long winded, but I hope that helps.