I asked this same question before we left.... we just returned from Disney and only participated in one EMH evening at MK on Wednesday, March 2. I would say the park was fairly crowded during the day, I found it somewhat difficult to navigate the crowd. However, after the fireworks, it was joy to ride the rides. We practically walked on to the motor speedway (that has NEVER happened) and had similar experiences at the Fantasyland attractions. Cast members did turn away quite a few people without wristbands... I'm sure they just "love" the whole EMH thing... but I thought it was great. My kids are little (3 and 6) so we weren't interested in the bigger rides, but my guess is the lines have to be much more manageable for those as well.
We split the day, spending the morning at Epcot, and then heading to MK for the evening. I think the crowds at MK were greater than on Monday (typically a high crowd day) but it was worth it to spend a couple of hours in the crowded park to enjoy EMH evening...