If your looking for a show that involves the whole 9 yards in regards story, characters, state of the art sound and lights as well as amazing pyrotechnics (fireworks) in an open stage then Fantasmic is the show you don't want to miss.
This is my favorite show of all time and have seen it every time I have gone (except for this one time that my girlfriend got sick and we had to go) . If you are going to visit the park with kids then they will love this show because at one point or another they will see one of they're favorite characters either ball room dancing on a boat or battling a giant fire breathing dragon on a island. This show has something for everybody.
If you want a really good seat then I recommend going early and saving your spot. Go as far forward as possible and try and get the spots where you have to sit down, especially if you have small kids with you. If you have to stand many people begin to put their kids on their shoulders and block the view for the rest. After talking with many of the disney cast members they say that the less crowded spots, although not centered to the island, are more towards The Haunted Mansion (In the picture on the right the Haunted Mansion is the big white building all the way at the left of the pic). Don't worry about not being centered as the show actually revolves around the Rivers of America and Tom Sawyer Island so where ever you sit, as long as its near the water, you can see the whole show. The boat in the picture on the right demonstrates how the show moves from one side of the river to the other.
There are two showings a night during the summer, and if there is more than 50,000 people i believe there is a third showing. If you go to the 8:30 showing it is a little harder to find a good spot because the park is still pretty crowded and you should get there at least by 7:30 or earlier and wait for the show to start. If you do decide to watch this showing then you have an option to stay were you are and watch the fireworks show that starts after Fantasmic over the Castle. Of course section in front of the castle is the best spot to see the Fireworks show but the rivers of America is just as good.
The show is completely centered around Tom Sawyer Island where the cabin is in the middle of the water. There are water projections all around so everybody can experience the show the same and the characters ride in boats around the river so everybody can see them. The reason sitting right in the middle is the best is because you have a great view of the cabin which is where the the show has its most dramatic moments.
I hope you love this as much as I do but I know your kids will not stop talking about it for weeks to come. =)