It's really a mixed bag.
For a handful of books, it has helped me big times. I recently read Pride and Prejudice with X-Ray and it did help keep the characters straight, even though I was busy at work and picking up the book only every 3-4 days.
But it is either off or very basic for most books.
I have a version of Adam Smith's Wealth of Nations with X-Ray and it's pretty much pointless.
While I would not call it a 'gimmick', I think Amazon's advertising is misleading. Sure, they make it clear it is not available on ALL books. But they don't make it clear that it is not available on 90% of the books. And, while you can see if a particular book has X-Ray enabled or not, you cannot figure out whether it is useful or not - the quality really varies from book to book.
Last edited by vxf; 12-07-2012 at 08:51 PM.