I usually stay on site for various reasons (I love the dining plan, not driving). We have not had great luck with EMHs on some trips. But this last trip we used them wisely and now I love them. If you can get up early, take a mid day break, and stay up late then they work great.
Go to park with AM EMH, stay for a few hours til park gets more crowded, take a break, then go back to park with evening EMH. Of course, there are not 2 parks everyday with EMH. So go to park with AM EMH then hop to park that does not have EMH. That first hour, and the next couple are usually great. And from about 11 on in MK with pm EMH is great. I couldn't believe how empty the park was after 11. We rode on Peter Pan over and over at 2am. NO ONE in line.
If you are not a family that can get up early, or stay up super late, then they are not worth it.