The main benefit to using Livemocha is that it is a community, who helps each other learn languages. So when I submit a voice sample in Russian, a native Russian speaker will rate it and provide me with feedback. I do the same for others who are learning English. It is also possible to engage in chats with users, and of course there are the lessons which are pretty good.
I have to admit, I'm surprised how good this site is. I also have Rosetta Stone, which is great and helps a lot, but one thing it does not provide is feedback from native speakers which is obviously helpful.
While I don't think that you can learn a language from one source alone, Livemocha is a perfect complement to other options out there, like Rosetta Stone.