I think it depends on when you go. Right now with June/July/early Aug being some of the busiest months, night EMH are just too crowded. When we went in early May, we could just walk on most rides, but the most popular (Soarin', Test Track, Space Mountain), the lines were too long to wait.
If you are going in Late Aug/Sept/Oct, I would still give it a shot. Just have a backup plan if it is too crowded. My wife and I plan to do Epcot 9pm-Midnight EMH while my parents watch the kids back at the resort. Our backup plan if it is too busy is to go to Boardwalk's JellyRolls, Atlantic Dance Club, or take a bus over to Pleasure Island. We will probably leave by 11 anyway and go to JR.