Yes, Fantasmic is definitely in the
do not miss category.
Depending on the time of year you're planning to be there, the dinner package can be very advantageous. We always use it. You're given a ticket that gets you into a special section of the theater with reserved seats. Some claim these aren't very good seats, but I disagree. It is off to one side of the theater, but I've never had any trouble seeing everything.
The three restaurants where the package is available are the Brown Derby, Mama Melrose's, and Hollywood & Vine.
We've never tried the Brown Derby (perhaps next summer). If you're on the dining plan, this is a signature restaurant which will cost you two sit-down credits each.
Mama Melrose's is an Italian restaurant -- we really love it. The only difference between being there on the Fantasmic package and ordering off the regular menu is that you have to do the pizza for your appetizer. (You share one for the group.) For
DDP, this is a one-credit restaurant.
Hollywood & Vine is a buffet. The food is great, but we weren't very fond of the atmosphere. The tables were very crowded together, and we found it very loud. For DDP, this is also a one-credit restaurant.