Book Trailers: yes or no?

I've read a lot of posts around the wonderful interwebz that talk about book trailers: short little video clips that summarize a book's plot in about a minute.

Personally, I've only ever watched a few, and they were mostly for books I had already finished. I liked a few, I disliked a few.

Has anyone ever watched a book trailer and then bought the book specifically because the trailer was that good? Has anyone ever watched a trailer and decided against a book they were previously interested in? What makes a good book trailer?

Do you think your book trailer has the right stuff to seal the deal and bring about a sale? Post it below and I'll embed it in this post!

Probably the best book trailer I've seen:

I'm not sure if this trailer is for a hardcore erotica or a crime-noir thriller, but regardless, I'm interested. Very good production quality as well.

Another solid noir style trailer. Not as good as the two above, but certainly effective.

Pretty sweet trailer from a new author

Very well made book trailer that captures the tone of the novel perfectly.