It pains me to give Rosetta a 2/5 star rating, but, I think it is justified. They obviously put a huge amount of effort into this product, and it shows. However, I do not agree with their essential assumptions regarding language learning are accurate. Rosetta is all about total immersion, teaching you a language through total immersion in that language, (in their view) simulating how your learned your native tongue as a child. This paradigm manifests itself in a key difference than how other language tools work - Rosetta is 100% in its target language (here French). Rosetta tries to teach you language concepts by giving you a limited number of learning concepts (say, for exe, vocab), and then requiring you to apply it to a given situation. For example, if its teaching you 4 words, you'll be presented with a screen of 4 images, each of which corresponds to one of the words, and you have to match word to image. So for learning vocab, its outstanding. However, the major failure is on grammar and usage. The same principle applies - it will present a grammatical construct, and use image mapping to teach you. The problem really arises on nuanced principles. When the verb is "to be", its easy to learn "he is..." vs "she is..." by looking at an image of a man vs woman doing something. If you take a harder construct, for example, how to form questions, it becomes very difficult for a set of images to accurately convey the real meaning. For example, asking a yes/no question involves "est-ce que...."; asking a "what" question involves "qu'est'ce que..."; I found that the images could not effectively help me understand the difference between these two constructs. The same applies to many other types of usage, such as the when to use il/elle est vs c'est. Bottom line is that you lose a comprehensive understanding of what you're learning.
In contrast, I have a lot of formal language training in Latin and Japanese. In both situations, you learn basics (e.g., verb conjugation), then apply them to situations. I think one of the problems with Rosetta is the paradigm - they assume that you can learn a language as an adult in the same way you learned as a child. I disagree. As a child, you have no learning framework yet, other than osmosis and mimicry. As you go through school, you learn learning structure and learn to hold information in a structured way in your brain. Rosetta doesn't do that.
In contrast, I just found the product Fluenz and absolutely love it. It inverts the paradigm and teaches French by relating it to what we know in English with good, highly detailed explanations of what and why.