RoomService 2K is a Chicago-based business, and not surprisingly, most popular in Chicago. If I were based in Chicago, I would pay to advertise there. There is a very good reason for the existence of this service, and that is that Chicago vice is rabid in their pursuit of working girls. It's been around for over 10 years, and I've never heard a bad word about the owner, although I have heard about cops managing to sneak in a few times over the years.
RoomService 2K might be the most useful for clients, in that once they have been vetted and become paid members, RS2K will vouch for them even if the escort is not a member.
Date-Check was started in the early aughts by a guy who was already somewhat known in the world of whoredom for his adult web services. He started with a good reputation that he slowly built upon by adding known, reputable escorts to his site who could vouch for new client members. I've never heard even the slightest rumbling of any impropriety. This site is more popular with East Coast guys.
Preferred411 is owned by a woman who apparently also owns an escort agency in Texas. It arrived on the scene a few years after Date-Check as a competitor. It operates much the same way as Date-Check, although IMHO the site design is a lot cleaner and easier to navigate than Date-Check. Some folks are naturally nervous about the agency connection, although after years of keeping my ear to the ground, I've never heard of any problems with the agency connection. Their servers are in Canada, and I believe that on paper (though I could be wrong), the whole operation is based in Canada. I heard one particular story of Preferred411 not thoroughly vetting a member who had given false work information. I do not know if this sort of thing is a common occurrence with Preferred411, or a glaring exception.
I know that Date-Check and Preferred411 requires somebody trustworthy to vouch for escorts that want to join the site. I'm not certain what RoomService 2K requires for an escort to join, but they asked me to confirm that I was a real escort with an established reputation before they would vouch for one of their client members.
All 3 of these sites present some risk to the members, just by their very nature. However, none of them are fly-by-night sleazy operators. In all my dealings with them over the years, and in all the years they have built their reputations, there have been zero incidents (that I know of) that would lead me to believe they are anything other than the legitimate businesses they claim to be. Membership fees are posted on their sites.