It's very possible to see a chunk of attractions during morning EMH. It also puts you ahead of the crowds for much of the morning; you can see a good portion of a park by lunchtime if you take advantage of it properly (and then leave once all the "I meant to get up and come but am here late" crowd, i.e. the majority of people, arrive).
Evening EMH, however? Slow at the MK in August? Whomever told you that either hates you, or is woefully misinformed LOL. A HUGE percentage of resort guests will end up at that park, much more than it would get on a normal night. Why? Because of the "I paid for this!" mentality, and the fact that most people make their park schedules wherever EMH is.
In slow season, evening EMH can be fun, especially the last hour. August? Wouldn't even dare.
It's pretty straight forward. Each day one park opens one hour early, or stays open three hours later (sometimes one of each, i.e. it could be AM EMH at MK the same day it is PM EMH at AK), for resort guests. In order to participate you need to be a Disney Resort Guest, and show your ID to get a wristband at the park in question.
The morning ones (AM EMH), *IF* you are actually there and walking in the park at 8AM, are blissful. The PM EMH's are a mixed bag...I enjoy being in the parks late for the atmosphere, but the crowds can get pretty bad for rides.
You can find the EMH dates for your vacation at disneyworld.com's calendar. Good luck!
AEfx