[mouly]

Usability is not a constant property. When we first see an interface we evaluate its perceived usability. Research has shown that this is mostly a function of its visual appeal. I think skeumorphic design has a high perceived usability. Someone who has never seen an iPad would recognize the iBook. But the same is not true for kindle. It does not look like a book.

After using the interface over time, our perception of usability changes. I suspect, overtime skeumorphic design will lose its appeal overtime because many constraints will be forced on the user artificially. Flipping pages on the iBook may seem more work than just tapping.