The food at the Sci Fi is pretty darn mediocre. That said, it's one of our must-visit restaurants.
We usually get the burgers (meat and veggie burgers are both available). Even though burgers aren't on the dinner menu, they've always let us order them any time of day. I had a couple of the entrees, and was pretty disappointed. The catfish was particularly bland and overcooked.
Sometimes people will eat dinner elsewhere, then come to the Sci Fi just for dessert. The desserts are pretty good, and you get to enjoy the campy atmosphere without paying for a full dinner. (Of course, this strategy doesn't work well if you're on the Dining Plan.)