IMHO - it's a mixed bag.
We just got back 3/2, and for MK NIGHT EMH it was well worth it. MK close at 10pm, EMH until 1am. Starting with parade times (another good time to hit attractions) we rode buzz about 4 times, space mountain twice, and then headed over to hit splash mountain and thunder mountain. By 12pm, we had also hit jungle cruise (9 people on the boat our 5 plus 4 others), and POC without any wait. Basically by 11pm, the park was almost empty (as empty as MK can be), and had no waits, with one exception - space mountain. That was because it kept breaking down!
MK EARLY EMH was again a success, as you can ride BUZZ and SPace mountain several times with minimal wait, and get FP's for later in the day. Head over to Thunder and Splash, and had minimal wait, just as the park opened.
MGM EARLY EMH was a delight - We hit RNRC, then got a FP, and went over to Star Tours. We got right on at Star Tours, and then headed over to Muppetvision (no wait). By that time, it was FP time for RNRC again, and went and did that. By that time, the full park was open, and we just went about our business.
MGM LATE EMH was much more difficult to call a success. I counted 8 attractions that are open during EMH, and let's face it, 2 of those have huge crowds - TOT and RNRC. We lined up for FP's at 7pm, and at 7:07 they opened the FP line. Our FP's were 7;45 - 8:45. By the time we made it back from Dinner at Sci-Fi, at 8:35, all FP's were gone. We did Star Tours, and just walked around. Not much else to do.
We didn't do AK NIGHT EMH due to last year, as again, there's not a whole lot to do early in the year. It get's dark by 6pm, so none of the "close at dusk" attractions are open. That bascially leaves Dinoland. Hopefully EE will help, but I think you are better off doing early EMH for AK.
EPCOT is similar - EARLY EMH you can so Soarin (get a FP first), wait about 5-10 minutes, and then ride your FP. Run over to TT, and either get a FP, or 10-20 minute wait. By that time, park is open, and crowds develop.
LATE EMH is not as successful, but still fun. Just remember FP's start all over at EMH, so if FP's are gone at 6pm, then the will start over later on during EMH. The other attractions during late EMH are the ones to hit (I shrunk the audience, figment, etc.).