EMH (evening) in the MK is not too different from the normal crowds, however, we found that some of the major rides such as Splash Mountain and Thunder Mountain were a lot less crowded. We seemed to see a lot of people walking around, but not very long lines.
I think the best time to go is not the EMH, but at around 9:00 PM. That's between Wishes, and Spectromagic, so the lines are VERY small. We went last July, and at around 9:00 PM we were able to ride Thunder Mountain 4 times in a row (could've gone more, but I was feeling a little quesy), Splash Mountain had little to no line, and when we went into Adventureland, NOBODY was there. we saw a cat lingering around, but that was pretty much it (besides the CMs). So we walked right onto Aladdin's Magic Carpets and watched Wishes from there.