I haven't, but I can definitely see an advantage if it's done well. A professional video would make your book look like it's coming from some big-name publisher with Hollywood resources. But maybe you need Hollywood resources to do it? There are probably high school kids these days that can make a 30 second movie on their iPad but i don't know anything about it. Would be interested to know, though...
Seems like everything I've read says that book trailers aren't effective. Personally, I've never been swayed by a book trailer and I've never seen one that was very impressive. Most are like slideshows, and who really likes slideshows? I don't know that a book trailer is even an appropriate medium for selling a book.
There is a company called Book Candy that produces beautiful trailers. However, they are expensive. The vast majority of companies I checked into produced things that looked like they used Power Point. Why pay for that? I made my own using Animoto. I have had plenty of positive comments but I'm not sure that it has helped with book sales. You can check out my trailer so you can see what you can do with Animoto. http://youtu.be/8FflybtiFMw They have stock music as well as images if needed. I have my trailer imbeded in my website, on my blog, on Goodreads and Amazon. If nothing else, it's a great creative outlet!
Effective?
Cost? $$$$