Hi There- I've used it quite a bit. It's pretty much the one-stop shop for film festivals, and the only way to submit to many of them. It's nice to be able to load up your information (and even a screener of the film) once, and be able to submit to many fests in a very streamlined way.
I love Withoutabox. It makes submitting to several fests really straightforward. From what I've heard it's a bit of a racket for the fests trying to be represented on the site, but it's great for those of us submitting stuff.
@Michael P. Heneghan : Didn't it cost you a lot of money?