Actually you get to order off the menu, but it is "limited" to an appetizer, entree, and dessert. All beverages are extra, and are probably not included in your travel package.
However, if the meal is included in the price of your travel package, by all means take it. The food and atmosphere are excellent!
The Fantasmic Package means you get to go the Fantasmic Show after your meal with no waiting in line. This is especially important after a long day in the park.
The Fantasmic Show is fantastic for all ages (just don't sit too far down or you may get wet- and since it will be the middle of November, it may be a little cool). It's one of Disney's best efforts.
At 90 days before your scheduled arrival at WDW, call for priority seating at the Brown Derby (unless according to your travel package, you call another number/party to set up the seating). If you can't get seating within two hours of the show, make the reservations anyhow, and when you're within 30 days of your scheduled arrival, try again for seating approx. two hours before the show. Needless to say, inquire what time the show is scheduled for before making the rezzies, since Disney does change the times based on the time of year and the time that the park closes.