we used it for the Fantasmic Dinner Package at Hollywood and Vine and declared it "the best buffet on property" as everything was hot and fresh and delicious and a huge wonderful variety...
we also used the DDP at Mama Melrose and were pleased with our meal...
we found it to be a "waste" of credits to use it for lunch at sci-fi as we just did not care for it at all... and have never tried Primetime...
Brown Derby is great, but that's two credits. My family has never thought much of the other dining options at the studios, so we usually go over to Epcot. If you are hanging around for Fantasmic though, that's probably not a great idea.