This is about a world. The world is a ring. Um.
As a staple in the classic-sci-fi diet, I can appreciate that this book's worldbuilding was groundbreaking. The Ringworld is ingeniously conceived and impeccably painted, the sense of scale is reasonably well conveyed, and the alien races are good fun. Unfortunately the plot (more accurately, the lack thereof) didn't grab me; there wasn't the same sense of awed fear as Rendezvous with Rama, neither did I care much for the human characters. There's some really weird unnecessary gender stuff which comes across as sexist (all the women in the book are plot objects who explicitly lack intelligence; the POV protagonist, a human man, whilst talking to an alien who is as absolutely unlike any human as anything he has ever met, muses whether women are of a different species to men. Um, you're talking to a species with two heads and no gender, you dimwit; spot the difference.).
Like many sci-fi classics, later writers have taken his ideas and redone them better (see also Banks' Orbitals, Halo, though both of those are an order of magnitude or two smaller than the Ringworld). As a result it's worth reading for the historic value but not for the narrative.