I did it for the first time last September. Mama Melrose.
Well worth it (but I like avoiding lines). Almost no waiting to get into Fantasmic! Allegedly, some people wait two hours for the standard seats. Also - when the show is over (which is after park closing hours) - its much easier to exit, thus no waiting for trams. I think we got better service at Mama Melrose because we had the package. The waiter was very attentive to details about the package. He allowed a lot of substitutions to the package. Even got a free champagne toast out of the deal (for our anniversary).
Minor disadvantages:
The first time you make a PS, you may have to accept a very early supper (before 5 pm). Its my understanding as the FDPs start to sell out that they open up later dinner slots.
The package includes a huge meal - and you probably pay more than you would if you ate a'la carte. Just go there hungry and it'll work out. If you eat big meals, then the FDP is actually cheaper.
Also, I don't think that the 50s Prime Time Cafe participates in this - so its one less restaurant to choose from.