Like all language programs, it's a complimentary tool to actually talking to people in the language you want to learn.
I used it for Mandarin Chinese (which I've now mostly forgotten) and I'll tell you that if you're going to use it, pace yourself. I studied 3 hours a day and forgot most of it by the time it got to the more difficult exercises (yes, Rosetta Stone becomes quite thorough and difficult as you progress), so no more than 1 hour a day would be your best bet, probably in two half-hour parts.
It teaches pronunciation, reading, writing, speaking and listening, doesn't tend to explain much in terms of grammar, more relies on you to pick up the patterns.
The end of module exercises simulate a conversation that you must fill in the blanks to, which are sometimes whole (long) sentences.
It's definitely useful but there are cheaper alternatives available, as you're looking at anywhere from $500 upwards for the full language set.