The journey that these books take you on absolutely astounding. The ideas presented on both the Rama spacecraft and the alien races encountered is some of the most fascinating science fiction I've ever read. The epic scale of the discovery of Rama as it's presented in the first book was absolutely mind blowing, as was the journey made by the characters in the following three books.
The first book was written solely by Arthur C. Clarke with the following three written by Gentry Lee using Clarke's suggestions and ideas. There is a big difference between the two writers: Clarke is all about the wonder and excitement of exploring Rama itself and Lee is all about the characters. Although the two styles compliment each other very well, Rama itself seems to take a backseat in the three following stories, which could also be read as one very large story.
Despite this being one of the most gripping page turners I've read in a while, it does have plenty of faults. Arthur C. Clarke has always (in my experience) had issues with characterisation. The interactions with many of the characters feels a little flat and his writing is much better when he's describing vast, technological marvels. Gentry Lee on the other hand is all about the characters, however I think he goes a little too deep into them and ends up with very uneven personalities who sometimes make inexplicable decisions over the course of the story. He's also a little too obsessed with sex, and tries to fit a lot of it in. But what he does put in feels passionless and unnecessary.
Despite it's faults, the Rama series is an amazing journey and one that's well worth embarking on.