the concept of Rosetta Stone is just like the concept of learning your first language, except the critical part is you are missing the element of 24/7 immersion.
Remember how you learned English? Progressively more challenging books with pictures describing nearly every action described. And you had your parents and teachers there to point out things and say what they were. Rosetta Stone attempts to boil all that down into a program, and succeeds. But it's going to vary by person though, based on how committed they are, and if they are taking steps to really let every sink in, or just doing what is necessary to pass the tests.
It can definitely help, and if you have other sources, it'll be an extremely beneficial addition to your studies.
Taking Russian courses, I absolutely hate the translation-based style of learning a new language - everything is compared to your language. And I'm terrible at learning languages. I haven't put much time into Rosetta Stone yet (get it online through the Army, sweet), but what I've used definitely helped, and you start seeing how different aspects of the language are applied when sentences are brought into the picture. Much better way to learn the Russian case system than through textbooks. Ugh.