We did the evening EMH at MK this past Friday 7/22/05, with mixed results. I guess I had been spoiled by the E-ride Magic nights last year, so I was a little disappointed about how crowded the park was. I enjoyed it for the most part, but DW says "Never again". I would say it takes about all three hours of evening EMH to do as much as you can do in the one hour of morning EMH.
In general, I would say to make sure you do the super-headliners earlier in the day at park opening or using fast pass. Space & Splash Mtns. were just as packed as they are at 2 PM. We balked the first time we got in the Splash Mountain line and hopped over the ropes to do something else. The wait for BTMR wasn't too bad, but right when we made the last turn in the cue they closed the ride. They said it was closed indefinitly, but we were welcome to stay in line - we left to do something else.
The positives were that it wasn't as hot as it is earlier in the day and most of the lines are much shorter. We basically walked onto with no wait: Haunted Mansion, Stitch, Pirates, Magic Carpets of Aladin, and then Splash Mountain when we got back in line at 1:55 AM. It also seemed a little less crowded than normal at park opening. I think most of the guests who could park hop started their day at other parks and waited to the evening to go to MK. We started at MK until about 1 PM, hopped to AK until it closed at 6 PM. Then we went back to POR for nap/swim. We tried to get back to MK in time for Wishes, but we just caught it from the bus unloading area.