I had to move to Germany with little notice for work, so I literally started using this as I was travelling to Germany and I had NO previous German experience, although I had studied Japanese in high school and Spanish in college. While I felt that the program was well-paced and taught the language basics well, it did not prepare me for those day-to-day conversations/phrases that I needed to know right away (not that it promised to--I'm just saying that if that is what you need, get something else for the basic phrases first!) It was about two weeks of daily study before I got to things like introductory conversations, shopping, etc. that I needed to know to be polite company.
Having said that (that if you need German IMMEDIATELY, supplement Rosetta Stone with something more conversational), I did find the repetitive and logical nature of this program effective. It focuses not just on learning words and phrases, but on learning how the language WORKS--sentence formation, conjugation, etc., by example not by lecture or brute memorization. You also learn through reading, listening, speaking, and writing, so it is effective whether you are an active, visual or verbal learner.
Like pretty much any educational software, there are things that are pretty ridiculous (WHY do you learn sentences like 'The horse drinks' before you learn things like 'My name is...'? And why must some of the people be so silly looking in the pictures?) but these are minor complaints. If you have the time (and the money) to do Rosetta Stone, it is probably an safe bet that you will learn as much as if not more than with any other program. I recommend that you stick with the 'suggested' course, not the more complete course option at set-up, because that had TOO much repetition and I ended up, half-way through the program, switching to the recommended program. Also, if you really want to learn the language and have more than a month to two months in which to do it, buy the combo-pack of I and II or I thru III. You save money (and that means less packaging and shipping, good for the environment). The only reason I can think one would want only part I (other than not shelling out too much money all at once) is if one just wanted to sample German to see if there was interest in going further.
So, in summary, good effective program but NOT going to immediately prepare you (in, say, a week or two) for living in Germany. But a good way to solidly start learning the language for full comprehension, not just for getting groceries and asking directions but for expressing thoughts.