We did the Fantasmic dinner package once, and we were let into the theater before the regular guests. I would say that everyone was comfortably seated before the rest of the guests were let in. And we did the later showing of Fantasmic and were allowed to sit anyplace in the theater we wanted, not just the reserved section.
If you are on the dining plan it is a fabulous value! Especially if its the free dining promo
we usually book Mama Melrose's fantasmic dinner pkg. because it come with an appetizer in the package. (now with 2008 changes you can't get appy's with dinner, but you can here)..
Then if it is low season and decide not to use the tickets for the show we just walk up and sit on our own...