If you are going this May, you may not have alot of choices calling this late. I'd call ASAP. The last two Mays I've seen people turned away at 50's Prime Time, Hollywood and Vine, Mama Melrose, and Brown Derby. Sci Fi was usually taking walkups, but the time would be a few hours away.
What I'm doing this May is leaving MGM to take the boat over to Spoodles, then I'll return in time for Fantasmic (I hope as the earliest ADR I could get was 5:00 PM, when they open). I really don't care for the dining options in MGM, but would choose Mama Melrose if I had to choose one.