Transformers: Revenge of the Fallen did great and made $201,246,000 dollars but still number two to The Dark Knight which made $203 million during 5 days. But keep that in mind it is not final result yet and by Monday morning we can have the actual result and I can assure you it will pass The Dark Knight.
The box office doesn't mean that the movie is good, it is a known fact that sometimes bad movies make great at box office and Transformers is just another example.
People watch this movie for variety of reasons, some wants to see the robots, some wants to see Megan Fox...I am not really sure about the last one! But I was talking to a friend of mine over the weekend and he wanted to see the movie for visual effects and computer generated robots and that is enough to go to spend some cash for it and that's how Hollywood make big bucks!