We were at the first EMH at MGM and wasn't impressed at all. We did Fantasmic and stayed until 9pm. Even at 9pm, there were still lots of guests walking around without wristbands. They could not ride any rides but they could still shop, etc... Our impression may not be in agreement with others, though. It was also the last night for the Osbourne Lights and that place was packed with people without wristbands at 8:30pm.
I agree with another poster on a different thread to avoid Fanstasmic on an EMH night. Do the rides!
I'm really not sure how Disney will empty the parks of non-resort guests. In all the parks, the shops are open after closing, so guests always stay after the posted closing time. I suspect it will be the same with EMH nights. For E-nights at the MK, you have a very small number of guests with the wristbands. For EMH, you have the potential of thousands of guests. Much harder to handle, IMHO.
I will probably like the program when I learn how to use it to its fullest advantage. We just felt like we were in a huge crowd with not much to do if you didn't have a FastPass. I anticipated a smaller crowd but it didn't happen for us.