Morning EMH is an excellent idea, especially for the busy attractions (TT/Soarin @Epcot, TOT/RNR @ MGM, and anything at MK), because most people can't get up in time. It's still crowded, don't think that you will be there by yourself, but what we usually do is head to the EMH park that morning, getting there before it actually opens, and then hit the most critical rides (or get FP's) first. Once that's done, we stay in the park until it gets too crowded, and then head to another one.
We went this past February, and found that the Evening EMH's were not as good. In the AK, only Dino and some of the Funland attractions were open, so it was packed. Dino had something like a 75 minute wait.
MGM, the FP's were gone for RNR, and we aren't big TOT fans, so again, not much to do. We find that most of the eating places were closed, or already full, so you can't do anything spontaneous (advance reservations are a must). It was nice walking around the park, as the crowds themselves were so much smaller than normal.
Epcot was also tough to get into the good attractions, TT had a 60-90 minute walk up, and FP's were for around 11:30pm. So we had to decide on weather to stay there, and wait in the standby lines, or wait until almost midnight. This was at 9:30pm. The other attractions that were open, such as Honey I shrunk the audience, and Figment were literally walk on. No wait at all. Again, the crowds were small, so with the exception of long waits at key attractions, it was very easy walking around and just "sight seeing". Eating was again marginal, at best.
MK was probably the best of all. While more crowded, there's just so many more attractions at MK. The biggest issue with the Evening EMH at MK was the HUGE bus lines after everyone leaves. It took over an hour to get back to the resort (AKL) after the fireworks.