We just returned from a 10 day trip and I was looking forward to evening EMH's because me and my kids are NOT morning people and it was really a struggle for us to get up and ready and THERE in time for the early morning EMH's in the past. And then back then the parks were closed too early for us and we'd have hours with nothing to do in the evenings.
Evening EMH's worked well for us- we slept in during the mornings and our days didn't even start at parks until after we'd had lunch (either at our resort or at the park we just arrived at that day). The first evening EMH we did was at AK and like others have said- it was majorly crowded by the time the EMH hours came around. We tried to "beat the crowd" but apparently so did everyone else! It was closing at 10pm and we went out to the bus line at 8:55pm and you wouldn't BELIEVE how long the line was for the all star resorts bus. It was atleast 10 people wide (after the queue part) and almost all the way back down the walkway to the bag check area! I'm not kidding.Apparently everyone decided to leave at that time.
After that- at Epcot, MGM and MK evening EMH's we stayed until the last minute. We found out that if they close at midnight with the EMH- if you are IN line even at that last minute- you still get to ride even if the ride line is 30 min long. We also found out that if you stay until the last minute, ride that last ride you were in line for, walk back to the front and get your stuff out of your locker, return your locker key, return your stroller- you walk outside and have NO LINE waiting- even for the All Star resort bus! That worked well for us- since we are nightowls anyway.
MGM wasn't that busy during evening EMH but I'm sure it had to do with all the rain we had right around the time evening EMH started. (Fantasmic was cancelled at 10:30pm also- bummer on that....) Epcot had some long lines during EMH hours but really the only OUTRAGEOUSLY long line was test track- so if you do that one before EMH you should be fine. (we saw a sign once that said the standby line was 230min wait and the line was all out in the center walkway there- it was a really long line. I couldn't believe people were going to wait that long for that ride!)
MK didn't really slow down until after midnight but that last 40min or so (before 2am closing) was great- no one at all... you just ran from ride to ride. Especially good if you're in Fantasyland where the rides are close to one another. We just ran/skipped/walked fast from ride to ride during that last almost an hour there. Then walked out- got a mickey bar on the way out- and again no line at all after returning the stroller and locker key.
This really worked well for us- but I realize it's because we are nightowls instead of early morning people. We also sometimes went to one park from about 11:30-5 or 6 ish then headed to the park that had the late EMH hours... we didn't seem to encounter any large groups of people doing the same (ie: no long waits for buses and entering the other park at that time). It was just so great to stay up late at the parks and then sleep late in the mornings... I hope they keep evening EMH!