I've tried it for Spanish, French, and Japanese.
I'm currently fluent in French and Spanish. Mostly because of that program. The only downside is it's very expensive.
If you're serious about it though, get levels 1-5 instead of 1-3 (if they have 1-5 available for that language).
example:
RS Spanish 1-3:
http://www.amazon.com/gp/product/B001AFFZM2?ie=UTF8&tag=clogui-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B001AFFZM2
RS Spanish 1-5:
http://www.amazon.com/gp/product/B00282UXC8?ie=UTF8&tag=clogui-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B00282UXC8
If you read the reviews there, the proof is in the pudding, the majority of reviews are good.
The key to using Rosetta Stone, is to use auxiliary resources also. So that means, along with using Rosetta Stone, try listening to music with lyrics in that language, or watch a movie in that language. I also strongly suggest you get a grammar book. Also for Korean and Mandarin - you want to get a book with the alphabet.
If you want to save some money, check out:
http://www.abclearnlanguage.com/url/spanish/rocket
They have a few languages. It's EXACTLY like Rosetta Stone, but only costs $99 for the entire program. In some ways it's actually better. They currently don't have a Korean option (but will soon it seems) but they have spanish and mandarin.
Either way you go, both programs are solid options and worth the $$$. After all, you ARE investing in yourself.