Well not taken the plunge yet and not sure how well providers will use them. First I am not sure but providers may have to have member to vertify your a member. Alot of them have there own screening methods. I seen a few providers websites which request certain information first find a provider you wish an appointment with and then contact her. From other post and what I she may request info like name ,telephone, work phone. Some if they have websites will tell information you need to give them.

Preferred 411 is pretty much Texas based but they are trying to get their name more widespread-

Preferred 411 is a very good site..
date-check and rsk2000 both work pretty much the same.. All depends in what state are you are in.. Some verification sites are more prevalent in other areas than some...
Rsk2000- Huge in Chicago
Date-check - huge in DC ( where the site was evolved) and all over the northeast.

Such as Ter is very popular for reviews in huge metro cities... other are great for tier 2, tier 3 cities....You may see a girl that isnt reviewed on Ter at all... but she has over 100 on another board... because that is a gents "bible" in another area of the country.

To sum it all up.. all depends on what part of the country you live in..