I was at Epcot this past Friday evening for Extra Magic Hours (March 17) and the crowds were, in a word, HUGE. Add to that, a breakdown or two and you really have a wasted several hours. For example, after waiting in line for about 20 minutes, for the Test Track, the lines quit moving (I was in the single rider line). After about 10 more minutes, a voice came over the loudspeakers advising us that the ride will be suspended for a while for the fixing of some problem or other. I left the area after that. As I was leaving, the previous announcement was being repeated, and the lines were not moving.
The wait time at Soarin' at about midnight was over an hour, and no one seemed to be going home.
HOWEVER, on Sunday night (March 19) the crowds were much more manageable, and I was able to do many rides and attractions between 10 and 12:30, doing some twice.
The only thing I did notice was that while the official closing time was listed at 10:00, the Cast Members were not checking for the wristbands at ride enterances until about 11:00. So the lines were a bit long until after 11:00.
My advice to you? Avoid extra magic hours, especially at Epcot. Visit the parks that are NOT having extra hours.
Enjoy.