This looks amazing. It was about time for the publishing world to bring disruptive ideas onto the table. From what I understand, you will be able to read your books on all devices which is definitely something that appeals to me (I hate Kindles!) Will explore the website a bit more - I see you can also read the first chapter for free.
I think the appeal of this is that the books are enhanced, visuals, pictures, diagrams in full colour and optimised for the web, etc. - something an .epub file will never achieve.