Will people please stop comparing this book to "Rendezvous with Rama"? Of COURSE the 3 sequels in this series aren't the original. Let's move past that.
When I read Rama II, I was disappointed in many ways, but pleased in others. ACC is fantastic at what he does, but character development isn't it. So I was happy to have characters with some sort of depth to them. Sadly, these characters seem to lose depth with time. (Aside: Why is it that middle-aged scifi authors always write about exotic women with French accents who just happen to be amazing in bed?)
As another reviewer noted, the "revelation" of Rama was a bit of a cop-out. That's okay, because I'd known for years that ACC and Gentry Lee had painted themselves into a corner with the Ramans. Nobody could have pulled off a genuine revelation of the Ramans. Really, disappointing though it was, a cop-out was the best way they could have handled the revelation.
So, if the characters were terrible and the plot disappointing, why 3 stars? Simply because it was a valiant attempt, as was the entire series. ACC never intended to write a sequel to "Rendezvous," and the book on its own did not deserve one. He presented us with a mystery of stupendous magnitude, but nothing else. The sequels are worth reading simply because they attempt to put the mystery into HUMAN terms. We might not like or believe the characters, but it's good to see that they at least have names.