By itself I don't think book trailers accomplish very much; they have to be coordinated with other publicity/marketing methods.
On the other hand, Amazon and Smashwords allow book trailers (with Amazon, you have to create an account on Author Central). So it gives potential readers a different kind of medium which can justify the purchase decision.
I have created one book trailer and am working on another. I have also studied other book trailers and have reached the conclusion that lowtech works best. Many of the book trailers seem way too polished and marketing-ish to convince me of anything. On the other hand, I have seen some book trailers in sci fi genre which definitely made the book more appealing to me. But sci fi is a special category, and the graphics/presentation for these book trailers were top-notch -- and expensive if you had to pay someone to do it.
In my opinion, book trailers are fairly good at building an author brand. You could just have a 2-3 minute video of the author speaking about why he/she wrote the book. If the book is nonfiction, the author could talk about what led him or her to write the book (that is always interesting). Keep in mind that viewers tend to judge authors by appearances or manner of talk or dress. Some degree of mystery can be good for an author.
Update: I just checked the youtube page for my 1st book trailer, and I see that in 2 years the video received 164 views. That's not a lot of payoff; on the other hand, a lot of self-help, business gurus do youtube videos to accompany their book and get a good number of hits -- especially if they are peddling practical knowledge.