Update: I got the Spanish version of Rosetta Stone about two or three weeks prior to me leaving on a fairly long trip to South America. In the end I don't regret it, but there are definitely better ways to learn a language. I don't know what your timeline is, but I had three busy weeks of preparations and work before leaving so I didn't have tons of time to devote to RS. From my experience it's fine for learning vocabulary but it doesn't seem to cover much in terms of verb conjugations. It also gets insanely repetitive. If I had to do it again I would take a crash course in the language, because once you have the very basics down the rest just becomes adding vocabulary. Ultimately the best way to learn is being immersed and surrounded by it like someone else said.
BTW, I know this thread is old but there still seems to be some interest in this so I'm replying anyways.