I would say the effectiveness of a book trailer really depends on the book and the trailer. And professional ones aren't always the best. Something that really shows what the problem is and what's at stake to the main character(s) can go a long way to drawing reader attention.
But are they watched and do they succeed in convincing readers to pick up the books?
While I've seen several great book trailers, the only one that's gotten me to buy and read the book is this one:
How about you? Do you find yourself picking up new books that you've seen videos for? Do you feel this is a good form of marketing when it comes to books?