Definitely a good book, though it starts out rather slowly. It doesn't really pick up until the Pak Protectors are mentioned and Louis Wu realizes that they built the Ringworld and humanity are descendants of normal Pak colonists on Earth millions of years ago whose Protectors died off.
Because it took so long to get to that point and thus to find the Ringworld control centre, the actual fixing of the Ringworld's orbit seems a little too pat and deus ex machina to me.
I give it 3.5 out of 5.