This may well prove to be one of the more controversial reviews on this site, as Ringworld is a staple of every SF reader's basic diet. But a little perspective is called for, I think. The Ringworld itself, a colossal manufactured world circling a sun, containing enough room to solve any planet's overpopulation problem, is a conceptual gobsmacker. It may well be SF's most amazing concept period. For its influence and brilliance of ideas, Ringworld richly deserves its classic status.
However, get past the Big Idea, and the actual story of Ringworld is a whisper-thin thing, a suprisingly low-key tale of extraterrestrial discovery in which not a whole heck of a lot really happens to grab you by the short and curlies, and with enough unanswered questions that Niven was essentially put in the position of having to write a sequel ten years later in order to settle them and stop his fans from grousing. (Later he'd turn the whole shebang into a franchise.) Ringworld is a good, but not great novel, that almost rises to its premise but runs rings around delivering full satisfaction.
Louis Wu, a disenchanted Earthman, is approached on his 200th birthday by Nessus, a quasi-equine alien known as a puppeteer due to the two flat heads it sports on lengthy necks. Nessus is looking for a small crew of aliens to accompany him on a deep space voyage, of the goal of which he's annoyingly secretive. The other crew are Speaker-to-Animals, a member of Niven's enormously popular feline Kzin race, and Teela Brown, a young Earth girl who is included for reasons that constitute nearly an abuse of the suspension of disbelief.
The puppeteers are fleeing a wavefront of radiation emanating from a chain reaction of supernovae that has occurred at the galactic core. Now, we're told the puppeteer species are primarily known for their timidity. At the slightest threat they curl into a little ball like a doodlebug, and they are so afraid of spaceships they have engineered their very worlds to fly through space. Niven here is straining credulity to the breaking point. It's not so unimaginable that a species could contrive to turn its own planet into a spaceship, but wouldn't a species that balls up at the slightest provocation do just that when presented with armageddon itself? Converting, not just one, but five worlds into formation-flying spacecraft takes just a little guts, grit, and determination, attributes Niven goes to great pains to assure us puppeteers don't possess.
Well, Niven solves this issue by breaking his own rules. Nessus has been chosen by the Hindmost, the puppeteer leader (and a witty title he has, too), to undertake this journey because by puppeteer standards he is insane: Nessus doesn't exhibit the fear response in the way normal puppeteers are expected to. So Nessus, Wu, and company head off into the blackness of the void aboard the Long Shot, a puppeteer craft powered by an FTL drive (Nessus has offered Wu and Speaker the drive's technology as an incentive to come), and in no time flat they arrive at the Ringworld.
The novel is enjoyable escapism up to this point, even as Niven is asking readers to cut him a bunch of slack. Once we are down on the Ringworld's surface, though, readers' expectations are defied in both good and bad ways. For one thing, it's good that Niven sticks to his hard SF roots and doesn't turn the story into a Star Trek episode once the characters are forced to crash land. The Ringworld is stupefyingly immense, a full million miles wide (roughly four times the distance from the Earth to the moon), and so our heroes cover very little ground as the story progresses. Much of the action of the book's second half involves a lot of zipping around on little personal flycycles, and our characters' only real goal is to find a way to salvage their spacecraft and leave. The absense of real suspense, an ingredient one might consider essential to any good adventure yarn, is conspicuous. Eventually, Ringworld inhabitants are discovered, living in the ruins of a once-great but clearly short-lived civilization. Niven devotes appropriate storytelling space to speculations as to why the Ringworld civilization failed, but the most maddening thing about the book is that we never really learn a thing about the Ringworld engineers of any significance. (Like, how is it that they are almost human!?) Which, as I mentioned above, is why Niven eventually had to cave into pressure to write a sequel.
Yes, there are wonders here. A brief detour by the characters to the puppeteer homeworld more than adequately evokes a sense of alien exoticism. As mentioned earlier, the Ringworld itself is the mother of all SFnal concepts. And our quartet of heroes are well-drawn. They engage us, if certain aspects of the plot do not, and we follow the story for their sake.
You should definitely read this book (hell, you should definitely read Niven period). Books that take both the Hugo and Nebula are generally considered sacrosanct, so I'm sure there will be many who disagree with my nitpicks. As always, your mileage may vary. It was my third time to read this book when I re-read it for review on this site, and my opinion didn't notably change, though some areas of confusion from past readings were cleared up for me. I did find myself comparing Ringworld to a similar novel, Arthur C. Clarke's Rendezvous with Rama. Both tales are about the discovery and exploration of a dazzling alien artifact, a self-contained world. Clarke, I thought, better conveyed that sense of otherworldly mystery, suspense, and wonder, though Niven certainly has him beat for scale. Thing is, size doesn't always matter.