I can only speak from my own experience, but I have done work for EM and they are indeed very selective.
I have known one of the partners there for something like 15 years and we had worked very closely together. Yet I still had to go through the screening process. And they did call up my references - they were very thorough.
In terms of utilisation, it's very dependent on whether your skills are in demand, and how available you are.
I have my own practice, so sometimes I get a call and I can't commit the time. Other times it fits in perfectly with a lull in my own workload.