Just to add to my prior post, someone who has never seen Fantasmic will probably not sense a real difference regardless of where seated.
But, speaking as someone who has seen the show multiple times, I really was disappointed with the dinner package seats. If you take one of the backstage tours or read into the history of Walt Disney World, one of the concepts often discussed is the importance of "show" in the guest experience. Every controllable aspect of a day at WDW is carefully choreographed by the staff to make for the most enjoyable guest experience. For example, there's a very good reason that you will never see a Space Mountain CM wearing a Tomorrowland costume walking through Fantasyland: To do so would ruin the "show" in the same way that it would be inappropriate for Buzz Lightyear to make an appearance in Beauty and the Beast.
Anyway, there was one part of Fantasmic in particular that really stood out when we were seated in the dinner package section. Toward the end of the show there is a large boat that appears and makes its way across the lagoon. The appearance of the boat is synchronized with the soundtrack--at a given point, the boat is illuminated and the entire stadium erupts in applause. Well, there are also pyrotechnics involved in this portion of the show. These effects were clearly evident to those of us on the far right of the stadium about 15-20 seconds before they were really meant to be part of the show.
This is a perfect example of what Disney would call "bad show."
We could also see performers moving around back there and, as I mentioned, we had a perfect line-of-sight for the Epcot fireworks. Next time someone asks for a great location to watch Illuminations, I think I'll tell them to try the reserved seating section at Fantasmic.
For those travelling in the summer months, try to pick a night that has two showings of Fantasmic and catch the second show. Even with the dinner package you MUST arrive 30 minutes before hand. At the 30 minute mark they open up the reserved section to anyone. Practically speaking, this means you need to be at the gas station entrance about 45 minutes ahead of time as it can easily take 10 minutes to get from that gate to the reserved seating section--more if you need to park a stroller.
On most nights with two shows you can arrive 30 minutes ahead for the second show and get a great seat more toward the middle.