This review will contain spoilers!
In many sentences:
I read the prequel to this book (Ringworld) several years ago, and only vaguely remember what happened in it. Obviously, this was before the book database. I did enjoy it, however, and was looking forward to reading this. Fortunately, for the most part, you don't actually have to remember the first - most of the important points are reiterated in this sequel. However, I do think I would have appreciated the growth of Louis and Chmee as characters if I had remembered more of their original adventure.
Upon reflection, one of the things I really enjoyed about this book was its ability to humanize the aliens that Louis encountered. The meat of the novel is, ultimately, Louis getting to know a small subset of the population. He doesn't meet but a tiny portion of the species on the Ringworld, but the ones he does meet are sympathetic. This may seem insignificant (and I didn't think much of it at first), but when you consider the fact that the Louis must ultimately make a decision that will kill 5% of the population, it is important that these deaths had weight. If we, as the reader, were simply told "5% of the inhabitants will die" it's easy to think we would not consider that all that great of a loss, even with a number like 1.5 trillion thrown around. However, when we consider that Vala, the giants, and the red carnivores will all die, it's much more powerful.
I also liked how Niven was able to balance the realistic aspect of the book (with such things as the mathematically valid data) with the need for an exciting, riveting climax. There were times when it was clear that Louis and co. had a year or more to try and save the Ringworld. Thus, they took their time to travel between locations and to investigate how fix the instability. At the same time, once they encountered Teela, though they technically had a long time, the intensity of the plot ramped up appropriately. Basically, I thought it was impressive how Niven could make a problem that they had years to solve exciting.
The whole Pak section was fascinating as well. The explanation for aging humans, and the idea that the presence of humanoids on earth and the Ringworld was due to a single source was logically satisfying. Rashathra, as a whole, was an odd aspect of the book. At first I thought it might just be the adolescent fantasies we found in Stranger in a Strange Land, but I think there was more to it. After all, the city builders controlled the population with sex. All of this, however, is consistent with the presence of thousands of different humanoid species that, while different enough not to produce viable offspring, are similar enough to allow for intercourse. Again, it's consistent with the universe that Niven created, and so it's not quite as arbitrary or fantasy-fullfillment.
I actually don't have any major complaints about this book. The writing was good, the story entertaining, and the sci-fi logically consistent and sufficiently nerdy. Obviously, this book is probably way to sci-fi for non sci-fi readers. However, if you enjoy the genre this is a highly recommended series and book. I actually hope there are more books about Louis's adventures on the Ringworld.
As an aside, the idea that city builders are extremely fertile, and will always get pregnant when mating, reminds me of the proverb that the unions of the gods always bear fruit that exists in greek mythology. I wonder if Niven envisioned the city builders as a representation of the Olympic gods? Or was this just a coincidence?