Will this service work on our servers?
Has anyone used it and if so what is your feedback?
They are just an automated nessus scan, really they charge a lot for a free product 
they tried to sell to us at last hostingcon as well.
IT does look way expensive, a client wants to use it. I told him I would look into it. What are some free ones that will work the same way?
none free, but lowest cost is trust guard
(i mean that allow you to put up a seal) Nessus can be had for free and is essentially what all these use, they may have some custom plugins but its still basically nessus vulnerability scans.