The biggest problem for me is the availability of Chinese audio books (specifically, non-abridged versions - anyone have recommended sources?) When your reading level is high enough to read novels, the English translation is no longer that important - if I could just get the Chinese audio and text I would be really happy. It looks like this is targeted at beginners. Not sure how helpful it will be for them, but I see a lot of potential for more intermediate/advanced learners. There's always that gap in language learning between text books and real texts that has to be overcome. I think this kind of service could be great for that.
Right now I'm reading Tian Long Ba Bu (天龍八部), and have gone through the first 200 pages like this (simultaneous reading/listening), and it's been really helpful.
Also, a feature request: give an option to display the text in Traditional or Simplified Chinese. There are browser extension to use this, but it'd me more convenient to not have to use it on every page.