I tried Rosetta Stone once. The only other language I speak fluently is French, so I will use it as the example. Rosetta Stone will give you the subjects, the main verbs, and some vocabulary. It will give you, IMHO, the equivalent to a the speaking capacity of native 5-year old. So, for example, the French Rosetta Stone will teach you the subjects (Je, Tu, Il/Elle, Nous, Vous, Ils/Elles), the principal verbs (avoir (to have), être (to be), Etc.) and some limited vocabulary (how to buy things, directional information, salutations, etc).
If you plan to go abroad with it, it may help as a tourist. Keep in mind, if you begin speaking in the language, some natives will assume fluency, and then go off on a whole bunch of phrases leaving you bamboozled. Most will realize you are not fluent and will help.
So, in my opinion, Rosetta Stone, while possibly useful, is an overpriced method that Sells itself as a shortcut; however, I think you will only feel you shortcut yourself. If you want to learn a language, nothing works better than a good old fashioned textbook, and, preferably, a mentor.