It depends. I've heard it's great on most languages but is worthless for Asian languages.
I used it a bit in a French class, and I hated it. It didn't actually teach anything. It just put four pictures of things up, with words under each, and you have to guess which is the right one. Even if you guess right, nothing is confirmed. You don't know if the word that goes with the pic of the yellow dog means yellow or dog.
That's not a big deal at first, because some of the stuff is easy to figure out (does this word that resembles "red" mean "red" or "dog?" I wonder), but once you progress a little, the damage done is great. You'll pick the right way to put together a sentence without understanding it at all. It's almost like learning the language by watching TV from the country. You might learn a bit, but it's mostly guesswork.
If you're looking into the Rosetta Stone for Japanese, I would say pass. I used it and it was absolutely terrible. It works exactly as ShaggyJebus said and it's worthless. Save your time and/or money and just don't bother.
ive tried using Rosetta stone for japanese, korean, and mandarin and it was an most epic failure it doesnt teach you how to read the characters and other basic things needed to know
they seem to have develop a learning system on what i think is an unproved concept of association. they give you the word in your native tongue, then show you the word in whatever language with a picture of it. im sure it tightens the learning curve but i dont think it revolutionizes the way we learn it either. the more recent editions of rosetta stone are to damn expensive anyways, ive seen some go for upwards of $500.
there are plenty of alternative solutions that are cheaper and more efficient ways to learn a language, many schools offer courses and there are plenty of lesser known language software that wont put such a dent in your wallet. some, like the one i have called 'instant immersion' cost me $30 and seems to cover everything rosetta does.