Rosetta Stone, well. I would say it might not be my favorite way to learn a language. To clarify...
We actually got two of them - Italian (1-5) and Chinese-Mandarin (1-5).
With the Italian, we bought it a few months before our wedding & subsequent honeymoon to Europe, which included a visit to Venice, Italy. I was unrealistically optimistic about the time I would have to actually get through it in the middle of wedding planning and didn't actually get very far, maybe through the first two chapters of book/part 1. What I didn't like was they didn't start with phrases I found useful. I learned how to say people were running or walking or maybe having a glass of water, but not where is the bus or where is the hotel or words for different foods, which is more what I wanted. The lack of more useful language knowledge didn't help me stick to it. But, if you have more time and aren't preparing for an imminent trip it might not be bad - the romance languages aren't that hard to learn for English speakers (I took 3 years of Spanish in high school and still remember a decent amount, which helped with the Italian.)
For the Mandarin - I took four years of Mandarin in college, so we bought that more for my husband. I found it was highly insufficient to get the sounds correct in the language. They have a feedback mechanism that would always say my husband was saying it right even though he was garbling things horribly. Not his fault, Chinese is a very hard language, but they weren't giving him good feedback. I spent hours upon hours in the language lab in college repeating things over and over and over again to get it right, and on top of that had class time where we were constantly corrected by our teacher, a native of Taiwan.
So, I would say the romance languages is a maybe - they are much easier to learn for English speakers, but any Asian languages I would take a class with a native speaker.