Honestly, the best way to learn a language is to immerse yourself in it. When I was in college I learned far more German when I spent a summer in Vienna than in the 3 semesters of classes preceding it.
That isn't always possible though so you have to find the next best solution for your needs and learning style.
I used to work for Rosetta Stone (as an assistant/photographer) and am quite familiar with their method. If you have some time and patience it isn't a bad method I just don't know that it's worth the asking price (to me).
They try to get you as immersed in the language as possible and all the audio is done by native speakers. All the text (other than very basic help text) is in the language you are learning. I do know that they also offer some sort of online community where you can interact with native speakers as well as have actual group lessons with a teacher.
If you can find it used for a good deal (don't buy anything older than V.3) it is probably worth it.
If it was a useless method the Army wouldn't have given them a contract to make a version just for them (which was insanely fun to photograph for a few weeks down at Blackwater). If you look through V 3 or newer you can see the back of my head in a few places. I even got face time in a couple shots for the military product.
*standard disclaimer* I do not work for Rosetta Stone and left an unhappy camper but am trying to give an honest opinion of their software.