There are certain advantages with agencies, once your trusted with them, it opens up all the girls that they have working for them, no need to be screened with every single girl and some agencies have 10 or more girls to choose from.
Also if you have an appt. scheduled and something comes up where the lady has to cancel, just call the agency and they will send another girl, no need to look around for a last minute appt.
I've had good luck with a well established agency