I haven't been to night EMH, but for the hard-ticket parties at MK...not much. And, they generally closed well before the park did (by 11pm when the parties went to 12, exception being Casey's on Main Street).
I too would like to hear first-hand reports, but for now I'm playing it safe by planning a good dinner well before EMH starts on those nights we plan on being there until Midnight. I figure we won't want to stop to eat that late anyway. However, if you do like to eat late, make sure you plan accordingly - most food back at the resorts is closed by midnight.
I made a point of doing the Fantasmic package on a night MGM has EMH, because I have a feeling the crowd for the show will be much heavier.
N.E.D.