Just remember MADE MAD:
Magic Kingdom - Sun
Animal Kingdom - Mon
Disney Studios - Tues
Epcot - Wed
Magic Kingdom - Thurs
Animal Kingdom - Fri
Disney Studios - Sat
Actual attendance at early entry can vary by season / park / resort occupancy levels...a lot of things you can't control. A lot of time there just doesn't seem to be much rhyme or reason. One person will post "MK was swamped during EMH", and later that week someone else will post "we had no lines for EMH."
We don't like to get up that early while on vacation, so generally speaking we avoid EMH. The best advice I've seen is to either avoid the EMH parks or to arrive pleanty early for EMH, with the goal being to "hop" to another park by mid-morning or lunch time. In ideal conditions, you'll have very easy access to the rides for an hour or two on the EMH morning, but after that crowd levels typically exceed what you would see on a non-EMH day.