Next to robust research, access to key decision makers will make or break your search. Being on the board of directors of an industry association or being viewed as an industry insider works to your advantage. If you are looking to speak with the best talent out there, and not just who's available the moment, the ESP's credibility should allow unfettered access to the A-Players.  In some cases, this "may" not be of all that much importance, but If not an absolute must you still should query the recruiter in regards to their knowledge of your company and your competitors.