I've tried both morning EMH and evening EMH several times, during winter time, spring and just recently in the summer.
My observations:
Morning EMHs are great because they're not too crowded. MK gives you the best bang for the buck, then Epcot, MGM, and then AK. You can pretty much finish off a lot of rides before the main crowds get there (typically around 10/11am). Morning EMH is so much better during value season when the crowds are low. In the summer, it's still good to do, but it tends to get a little bit crowded.
Evening EMH. This one is interesting. I find that during the low months it's actually nice to have evening EMH because the park hours are shorter. EMH gives you a good amount of extra time. However, because its in the evening, the park has had a chance to get the crowds built up, so it tends to be a bit more crowded. This is especially true during the summer when the one-time visitors see the perk as a way to maximize their dollar expenditure.
I've also gone to EMH parks the day after their EMH and found that the park was slightly empty. This is especially true for MK after evening EMH.
It's really a preference issue. When we go to WDW, my family will do morning MK, morning Epcot, skip morning MGM and AK. We tend to skip evening EMHs except for MK and only during low season because usually MK closes real early and it's a good way to maximize park visit.