If you've used translation services before then you're probably used to seeing the text in 2 languages in 2 columns. Or you might be used to using Google Translate, where the source appears when you hover the cursor over each sentence.
DoppleText does something different. They take the source and, one sentence (or fragment) at a time, they insert the translated piece as a window next to the relevant piece of the source text. (It covers up other parts of the source text, and that is slightly confusing.) Each source segment is highlighted as it is being translated, and you can advance through the text with the arrows keys.
The service launched back in October (in German), with the English language service launching in January. Right now they're offering just a few pd titles, and they're using professional translations to provide the second text. But DoppelText is also planning
If you'd like to check it out yourself, there's a number of free excerpts as well as a free Epub you can download. As for me, I'm not all that interested. It's interesting on a technical level, but I used it and I found it to be too confusing.
I also think that going sentence by sentence is the wrong approach. I look at translated content all the time (web, not books), and I have found that I get the most value when I look at the text a paragraph or more at time. I've found all too often that authors don't finish a thought in a single sentence, and to really understand them you need to look at larger pieces of text.