We have done the morning EMH on every trip and I think it is an absolute must. For example, during our first trip to Magic Kingdom,were there when the gate opened to resort guests for EMH. We were able to do Dumbo, Peter Pan, Snow White, Winnie the Pooh and Space Mountain before the gate opened to the public. We then jumped on Buzz and there still wasn't a line for Stitch, so we jumped on. We were able to do seven attractions and the public had only been in the park about thirty minutes. IMO, it is the greatest perk to staying in a WDW resort.
We have never done the evening EMH because we have not been since they started this, but based on what everyone has said on these boards, if you can stay for the last of the three hours, you will walk straight on the rides. If you can't stay that late, you will have additional time in the parks, but the lines will still be very long. We are going to WDW for two full weeks at Christmas. DD and I are going to stay to 3:00 a.m. one night, just to be able to say we did it.