Enterprise promotes an atmosphere that they will take any reservation whether or not they have a car (See Seinfeld). They also do not have set rates and you rarely get the same quote, try calling the Enterprise down the street as you will get a different rate if you want to see for yourself.
I rent a lot and now go to Priceline or Hotwire. I can typically get a car for about 17 per day. I would stay away from ERAC unless you have no other options.