I just came across an interview of Wowio head Brian Altounian. He demos one of Wowio's enhanced ebooks and briefly discusses Wowio's plans for the future. Watching this video reminded my why I stopped using Wowio: some of their ebooks have ads. And they're based in PDF format, and don't fit my screen.
If Wowio is serious about pursuing enhanced ebooks, then that last point is going to hurt Wowio in the long run. There's only so much that you can do to enhance a PDF. All of Wowio's competitors will be able to create a much more interesting ebook and leave Wowio behind.
Update: Brian noticed this post and pointed out that I had a factual error. Not all of Wowio's ebooks have ads. He also made a good point about PDF:
As for the pdf format and whether or not they fit your screen, WOWIO books have been made available to a global audience and WOWIO books have been downloaded by readers in 218 countries - the pdf format is still the universal standard and enables that widespread delivery. We also work in other formats but until the demand is there on a much grander scale, we'll stick to what's working for us an then migrate to meet the demand.
I don't care for the format, but he's right.