We use evening EMH all the time. We just got back from a 3 week trip, and have taken 3 other 2 week trips in early December in recent years. Our past experience was that EEMH were terrific and we enjoyed how the atmosphere of the park was so different at night.
However things have changed in recent months. Before then, the policy was that Disney resort guests got wristbands for EEMH, so that CMs could tell who was actually "supposed" to be there and who was allowed to do rides/attractions. While there wasn't a hard and fast effort to kick off-site guests out of the parks, you generally had to show your wristband to ride anything.
Now the wristbands are gone and you have to show your KTTW card on every ride/attraction you want to do. Not such a biggie on it's own, except now it seems that Disney is actually encouraging off-site guests to stay in the parks during EEMH to eat/shop. While they technically can't do any rides/attractions (and they're being fairly strict about this from what we saw) it does mean a lot more people in the parks than we've seen in the past. And forget those "whole store to yourself" shopping trips along Main St. in MK.
Another downside of the new system is that it's hosing up ride lines because either people don't know they need to have their KTTW cards out to be checked and have to go searching for them at the queue entrance, or CMs are having to explain to off-site guests that it's EEMH and they can't ride.
So.....
Bottom line is that once this little snafu gets cleared up, I'm hopefully EEMH will go back to normal and be the fun time it always was!
Oh, but one more thing. Understand that not everything stays open for EEMH. Pretty much all the big attractions do (which is great - when do you ever see Peter Pan or Snow White as walk-ons?!). Also food choices are very limited in most of the parks, with many of the retaurants/CS locations closing up at "park close".