I think as same as goliso. For absolute beginners RS is a great way, because they can hear the words properly. So, for the first steps is great. But, everyday speach not even close to that! All my housemates were Korean before, so, believe me, there is so huge difference.
So... all I can say... For the first steps rosetta is good. But after one's ears getting used with korean words, please change...
I find KC good, and like it so much, I have it on my iPod as much as I can.
My opinion anyway is, that having Rosetta "just in case" is good, near KC101, or any else learning stuff. But itself alone teach a "sterile" language, what frankly, will worth not too much, if someone goes and lives in Korea.