Lots of people misses the point with calling this book outdated, and even random. What the author has to say about the human brain itself can be purely wrong, it doesn't necessarily matter in the sense of human intelligence, because this book is about human intelligence, not the human brain. Also note the title that clearly says "speculations".

I'd even say, that this book is a rather fun and easy read about the concept of intelligence in general. Through several biological and linguistic phenomenons Sagan introduces the readers to an unbiased view of human intelligence, that can and should be compared to for example animal intelligence. Even when he talks about the estimates of some information specifications of a human brain, he is comparing human intelligence to machine intelligence.

This book may be light on the side of science but it's a really great introduction to the concept of intelligence in general. What I did enjoy as well is reading how empathic and reasonable he is with every matter covered in this book.

It has interesting or even weird ideas (like cultural myths encoded in our genes) that could be classified science-fiction but the point is getting rid of our biased view on human intelligence, looking at it in a more general way.