I'd never call myself a "linguist", but I do think I have a pretty good idea what it takes to really learn to speak a language. Way back in 1986 Uncle Sugar spent a fair deal of cash to teach me to speak russian. We spent 8 hours per day, 5 days per week for 47 weeks learning to speak, read and write the language.
I think that no matter how good a book/tape/dvd/cd or software is, without someone who speaks the language fluently (preferably a native speaker) to work with - you will not learn to actually speak it. At best you'll learn some canned phrases and replies. I've used Rosetta Stone and I think it would be an outstanding refresher or supplement to formal instruction. I do not believe that it will enable someone with 0 language ability to become a proficient speaker of the language it's teaching.
I've seen the commercials that say that the .mil and .gov use Rosetta Stone for language training, but I'm going to go out on a limb and say that they're probably using it in conjunction with an instructor who already speaks the language, and/or using it for language maintenance and refresher training. I doubt they're handing it to folks and expecting them to "pick up" a language in an hour or two each day.
tl;dr - Rosetta Stone is pretty good as a supplement or refresher, but seek out instruction if you really want to learn to speak a language.