I've seen Fantasmic! multiple times and I do love it, but we have skipped it the past few times.
The problems with it are this, for us:
-dining options at DHS....as in, there aren't any that we are interested in so we'd have to eat somewhere else early and then hoof it over to DHS
-crowded, crowded...there is always a line early to get into the theater and then you have to sit and wait on stone benches for an hour before the show starts
-"black clouds of death"...it never fails, when we schedule F!, we get to DHS and the black storm clouds come rolling in. We've had downpours, lightening, etc that has ended up cancelling F! after we had been waiting for a long time.
On the plus side, I love the show, I love the music, and it does kill me every time we miss it. I just can't get a schedule going where it works for us.
Also, we usually do a few days at Epcot because it does get tiring, plus it's got the most restaurants. We usually only do a long morning at AK. We get there at opening (sometimes EMH) and stay until about 1 or after lunch.
I have gotten to the point that I decide where we are starting the day and where we are ending the day and whatever happens in between, happens. If we start the day at one park and either get cranky or do everything that we wanted, we move on.