Science Fiction is the biggest genre in Hollywood now. Include a liberal idea of fantasy movies (in other words, allow movies based on comics), then there is no genre that can even come close in terms of box office success.
Look at some of the biggest Hollywood hits last year:
Revenge of the Sith (sci fi)
Chronicles of Narnia (fantasy)
Batman Returns (comics)
War of the Worlds (sci fi)
Charlie and the chocalate factory (fantasy)
King Kong (fantasy)
And fans of this genre are so devoted that film-makers working in other genres must really envy James Cameron, Spielberg, Ridley Scott, Lucas and such. Take a look at what one sci-fi fan has to say. Click on the link. Makes for interesting reading.