Off topic here, but I find these rather obnoxious. They're almost like mystery meat navigation in that they're state isn't consistent. You have a disappearing navigation element, so a user might see it at one point, misunderstand what causes it to appear, and then not be able to find something on the page that they saw before.

On the other hand, they do grab the users attention, but I wonder what the ratio of successful clicks is to frustrated users.

Just my two cents, and pointing out that at least some users don't think they're neat.