I've made it through the first three sections of Level One and I'm enjoying it a lot. Rosetta Stone is very easy and intuitive to get into - you simply match phrases with pictures and, by using the visual associations you've already made, learn new material. No language other than Hindi is used anywhere in the interactive lessons.
You should definitely learn the Devanagari script before you start, because RS does not use transliteration. While it does explicitly teach you certain Devanagari characters along the way, the pace is extremely slow. I feel you'll be able to make associations between characters and the spoken sounds in RS much more quickly if you already have some familiarity with the script (I recommend Rupert Snell's book).
The program is visually appealing, with lots of quality photography and audio - it seems like there are at least two different male and two different female native speakers on the audio, which gives you a broad variety of pronunciation references. It clearly took a lot of effort and money to create this product.
However, if you're not careful it's easy to coast along on the beautiful surface of the program, and not learn as much as you really can from it. For example, in order to teach you colors, the program relies on your prior knowledge of other vocabulary, so it might show you four pictures of objects and give you a sentence such as "the apple is red". It's really easy to click on the picture of the apple because you know the word for "apple", and completely ignore the new word "red" that they're trying to teach you. Even if I'm not doing a "speaking" section, where my voice is being recorded and analyzed, I still like to repeat all sentences and really study the text.
The default pacing of this program is too fast, I think - it always wants to move you relentlessly forward, but it's important to pause once in a while and really look closely at what's on the screen. For instance, sometimes you'll be given four pictures, and hear audio of a sentence (but no text) and have to choose a picture. Only after you've finished the screen does the text of each sentence fade up next to its picture - and it stays up only several seconds before the screen fades out and the next one comes up. I find myself scrambling to hit "pause" so I can really read the words and see how they're spelled.
For me personally, the intuitive approach is a great way to be introduced to new material, but in order to really learn the structure of the language, especially grammar, I've been looking at other, more traditional sources, such as Snell's "Teach Yourself Hindi", that lay out the rules rather than have you guess them.
I also find that once I've gotten through a section, it's almost impossible to refer back to a specific screen to refresh my memory, because sections are named very vaguely, and individual screens are only numbered. For instance, if you wanted to revisit the screen where they first introduce the words for the rooms of a house, it would be a frustrating exercise in trial and error. For this reason alone it's a good idea to have a Hindi-English dictionary and a grammar book or more traditional course on hand as well as RS.
Although it's hard to do your own review of specific words or rules, the program does automatically take you through review screens of previously completed sections, based on how many days have passed since you first completed them, which is a useful feature.
Overally I think this is a great product, especially for visual and intuitive learners. Rosetta Stone is unique in its approach, and it's a much more engaging and "entertaining' way to learn than many others out there, but most Hindi students will need other resources as well to really learn the language.