Whether or not it is worth it is sort of in the eye of the beholder. I've done the dessert seating several times, because I am unwilling to fight the hordes of humanity to get a spot. So if I don't pay for the seating, I don't see Fantasmic. As for the dessert, you get a box that contains cheese and crackers, grapes, a couple of chocolate truffles(white and dark) and then usually a little cheesecake, a lemon item and a little chocolate cake and a cookie. It's a lot of food. We usually snack on it and then take it back to the room (cheesecake is the breakfast of champions!)
A couple of things to note though. If you want a front row seat with no one in front of you, you have to line up at the podium by the churro cart in front of the rivers of America(near HM and the ferry boats to TSI

L) and you need to line up around 6 or 6:15 pm but they won't check you in until 8:00 pm. There are only about 12 seats that are in the front row, where the only obstruction is the wrought iron fence. If you don't care about being in that front row, you can show up around 7 or 7:30 and you should be ok.
Overall, I think it's wonderful and will likely do it again when Murphy the dragon is up and running, but it is not a perfect set-up and there is waiting involved (there are no seats to sit in during that 6:15 to 8 pm wait.) I just usually send DH off to ride some rides I don't like (Indy) while I wait and read or play with my Nintendo DS.
Have a magical trip, whatever you do,