It depends on what you're looking for:
Hollywood and Vine: Character buffet (depending) with food that I've found to be the worst buffet food I've personally eaten at WDW
Brown Derby: Most pricey of all HS restaurants, but very tasty food and unique desserts (usually).
Prime Time Cafe: Unique experience (unless you get a dud server) and above average food for a theme park experience. Kitschy with a good peanut butter and jelly milkshake (if you like that sorta thing).
Mama Melrose's is okay in a pinch but I've had much better italian at Olive Garden (and that's not saying much in terms of being any better than home-boiled box spaghetti and jar sauce). No real great experience any of the several times I've eaten there.
Even though it's the coolest atmoshphere of any HS restaurants, Sci-Fi Dine In has mediocre food that is usually not as good as a Johnny Rockets. Since it is pretty cool visually, this place is doable but I wouldn't go here just for the food (I would be disappointed). Of the three times I've been, I once ate the chili which had the largest and chewiest beans I've ever had (the beans were literally as big as the head on my spoon), I had a raw hamburger that leaked blood all over my plate when I cut it (this place is not one where I'd ever eat rare meat), and the third time my burger was so greasy my hands were coated in the stuff. It looked like I had just given a lube job to a deep fryer.
Bottom line, if you want pretty good food and a unique Disney dining experience, go with Prime Time. If you're looking for the best HS food, go to Brown Derby. If you could care less about the food and just want a cool experience go to Sci-Fi and unless you're set on characters, avoid Hollywood and Vine imho.
Oh, and if it sounds like I'm picky, I promise I'm not. I love to eat. I'm an eater. I love food. More than people (usually). And I'm all down for some mediocre meals but I would prefer to have those at home where the price is right. When I'm on vacation paying an arm and a leg, I'd prefer to spend even a little more for some good grub.