What do you think of “trailers” for books?
Personally, I think a well done book trailer looks nice and is a pretty snazzy component to use in a book's promotional plan, but only if done well.
They also seem to me to be the kind of thing that if you need one to help sell a book, then even a good one is not going to help you build the kind of audience required to really work a book trailer through. And if you're the kind of author who doesn't really need a book trailer then you probably will get a great one and see all kinds of amazing throughput with it.
In other words (if I may use a video game reference since I've been watching my kids play shooters a lot lately), it's a good weapon for the last guy in your strike team to have available, but I wouldn't think it does much good for the key folks on your team who are going to be leading the attack.
That said, I did produce one to accompany The Ruby Files that I think captures the spirit of the book quite nicely.