It depends on how you look at it, how early you can get up, and what your goals are.
For instance, if you REALLY REALLY will get up and be at the park by 8AM on an EMH morning, then go for it. They are wonderful. The crowds do build up as the morning goes on, though, because many inexperienced people plan to get up, sleep in, but still go to that park that day. If you REALLY REALLY get there early, by the time those people start to arrive (10-11AM) you'll have seen half the park, and even when they start doing their first attractions near the front of the park you'll be in the back of the park where it's still relatively quiet.
Then, around lunch, head out for a break or to another park for the rest of the day. That way, you've enjoyed the EMH park, gotten the benefit, but then get out of there before it gets really busy with all the "good intentions" people who slept late but still went to that park that day because it was in "the plan".
Now, as to evening EMH, they have a bad rep because they don't always mean short lines. You will find that pay-sites and books will tell you to avoid them like the plague. Many of us have a more balanced view of this.
Remember, those plans and publications assume your goal is to totally avoid any crowds and your only goal is to ride rides in quick succession for your ENTIRE trip. It doesn't take into account how, for instance, in many times of the year when WDW is slower and hours are reduced, it's the only way to hang out in some parks after dark. And I wouldn't dream of missing an EMH PM at the AK - even if you only do a few rides, it's SO awesome to be in that park at night.
Finally, regarding PM EMH's, you'll find many people say "Ugh, it was crowded, so after two hours we gave up." That's exactly what happens, and if you spend the first two hours walking around and enjoying things at a leisurely pace, avoiding crowds when you can, that last hour can be MAGIC. I've done things like ride ToT or Splash Mountain a half dozen times in a row, or stayed on the Haunted Mansion for five rotations. The parks really close out by the last hour, and remember : as long as you are in line by, say, 11:59PM if the park closes at 12:00AM, you get to ride - the rides don't close at exactly the time the park offically closes, that just means they close off the line. I've ridden a ride as much as 20-30 minutes after offical closing before.
The best advice, in my opinion, to examine what you want out of each day. For many, it varies. Although the popular wisdom seems to be "avoid EMH", if you are willing to work with them (and have a park hopper), you can really take advantage of them and make them as special as possible.
NED
