As @biggie eluded to, they are generally seen as a waste of time/money (should you decide to pay for one).
There is no one set standard or baseline in which to gauge a value of a domain name.
Its not like cars where the manufacturer sets a base price or a known commodity in which the global market agrees no a base price.
Domains are unique in that they are non-tangible, somewhat intellectual, a bit of a luxury, and have a defined scope (depending on the name).
That is my long way of saying that the price you sell a name for is the price that the name is worth.
Use the free estimates for the information they provide to get an IDEA of the value of a name but there is no one stop, end all be all appraisal system.
Just my thoughts... maybe someone else knows something I am missing.
Yes, on NamePros there's a subforum for casual appraisals by users, which is unlimited. I almost never venture into that forum though because a lot of the domains people are asking about are just scary.