There are several languages, including my favorites, Spanish and Japanese, available to study. The website breaks lessons down into slides (sometimes over 100 per lesson) that have a given phrase or conversation in the studied language with an English translation as well as an attached audio file that has the spoken pronunciation.
From the Japanese lessons I've tried, it sounds as though they have tried very hard to have native speakers voice the audio recordings, giving correct pronunciation and intonation.
The lessons are well designed, however, even at the basic level, the system expects one to have a working knowledge of the written forms of the studied language. At least, in my case, even the beginning slides were presented with the Japanese written in kana, which while no problem for me, could potentially be discouraging to absolute beginners.
In general though, Mango looks like it could be a valuable new service for anyone wanting to study a new language or brush up on a familiar one. Recommended.