I am interested in finding out if anyone has used MXGuardDog.com or cares to review them. I am looking for a better spam filtering solution than just what is available to me on my shared cpanel website.

MXGuardDog offer free MX spam filtering service in exchange for text links on my website. They say that they do not insert advertisements into the emails. How are they in business if they just want text links on my website and don't advertise in my emails?

Does it work? Is it reliable? Are they trusted? Is it better than I can do with my cPanel account and Spam-assassin? Are there other mx spam filters worth comparing?