I find it very useful. Usefulness varies considerably from one book to another, however, since the information is essentially crowdsourced. If you are jumping back into a book after some time away, it is great for refreshing impressions of this or that character.
Note it also compiles information from Shelfari, not just Wikipedia. It is not just the content that is useful, but the navigational capabilities it adds to the book (you can click on the graph to jump to other entries and then to another location in the book). It is not the same as a professionally indexed book but indexes aren't really supported in ebooks very well yet, and typically they're only available for non fiction books.
I have not tried Xray for textbooks, but seems like that would be useful as well.
They should add the graph navigator thing to the notes and marks list as well because as it is you may have many pages to swipe through to get to the one you want to look at.
Last edited by tomsem; 12-07-2012 at 02:30 PM.