We did evening EMH at MK when we were there in Sept. We arrived a little after 8:00 when SpectroMagic was going on (not by plan, but dinner took longer than expected). We followed the crowd down Main Street and we went straight to Adventureland. We rode JC, PoTC, Splash, BMTR and HM with no waits at all. We then walked over to FL to see if anything was worth waiting on and it was a zoo! I guess we were just going in a different direction than everyone else.
We really liked it, but I must also say that we are going to skip it this year and try a day with no EMH to see if the crowds are any lower during the day. Everyone says there is a big difference.
I am sure May is a little more crowded than when we were there, but if you are interested, I say try it. Just maybe don't plan on TL or FL, which is where it seemed everyone else was the night we were there.
HTH