I've actually tried these types of services with a couple of different types of web sites. The sites that offer 10,000 visitors for $29 are just driving traffic through pop-ups, etc, and other generally annoying methods, and are a complete waste of money. I've never had a single sign-up/sale/or anything else from this type of service.

There are some services that will send visitors via paid-to-read email though, and these ones can actually bring decent visitors, but only if you have a site where you are giving something away. The last time I used this type of service I think it cost $15 for 1000 emails.

If you have a site that gives something away (ie you're working on building a list by giving away a report or something) $15 is way less than you would pay for traffic from Google, and sign-up coversions won't vary that much.

Cheers,
Trevas