Hi PolyAddict,
We just got back last week, Fantasmic was wonderful!! Our experience was that at the 8:30 show, they announced standing room only while we were walking in (at about 7:30). Since friends of ours saw it that way (standing) the week before and their kids couldn't see at all we turned around. Luckily it was an Extra Hours night at MGM so they had a 10:00 show for resort guests only. There was a LONG line at 9:30 before they opened the gates but we got in with no problem at all and at least at this show, the end sections on each end were empty. I don't know if there was a reason for it or if it was just not that full. Anyway, it was well worth the little bit of trouble. BTW, TOT and RnRC had much shorter lines during the show. I second the statement that it is not a fireworks show, it is much more (lights, water, characters, songs, etc.) My 11 yr old didn't want to bother seeing it but boy was he happy we made him afterwards.
Have fun,