Most of these blast services cater to the small produstion companies, indy managers, small agencies and such. There are a few large companies they send out to though but I have never seen a really big success story-- like Paramount or Warner.
Don't get me wrong, there have been success stories and some people swear by these services, but from my experience, they are the same odds of success as doing it yourself.
My advice is to get the hollywood directory on disk, print out labels and do it yourself. Sure you're spending more, but at least you know who your'e sending it to and can followup later.
I did it, full out and slick, with a query letter along and a self addressed postcard (one I designed tastefully to fall in line with the theme of the script.) It had a yes or no interest checkbox on the back. I mailed out 450 letters, got 110 postcards back with 12 requests. Didnt sell the script, but made a contact which turned into a rewrite for hire. (more than paid for the letter campaign).