I think this can be a good idea if:
- your publisher does very little in term of publicity.
- you have a bad or non-existent publicist at your publisher.
- you are prepared to never, ever know if it is worth it.
- you can afford to drop the money.
I did it once, with Learning to Fly. It went into four printings, got reviewed all across the country (and even in Canada), was a Booksense pick, and got a movie option. I have no idea if any of that came from the publicist. It was also a good book, and I was responsible for that part.
Read about one writer's experiences here.
What do YOU think?