Sci-Fi is located in MGM. If you don't made an ADR, you won't get a seating. I planned a very last minute trip last month. My ADRs were made two weeks before we left. The only reason I was able to get a seating at Sci-Fi is because I agreed to split up my family into separate tables (knowing full well that once we got there they'd seat us all together since it was just me and two kids).
Portions are very large (you'll leave stuffed) and the servers were great. The food is acceptable, but if you go there expecting a meal like you'd get at Le Cellier or Coral Reef you're going to leave disappointed. Many threads at this board are filled with complaints about the food at Sci-Fi. I personally have no complaints about it at all...it just isn't gourmet like some of the other Disney restaurants. The atmosphere of Sci-Fi is great and if you have kids you should probably do it once. It was definitely a fun experience.