The premise of this book is fabulous. Take all the things which we've read about and seen in science fiction books, TV shows and films, and examine how possible, or impossible they are.
So we have phasers, death stars, time travel, warp engines, telepathy and many many more. Yes, it's a geek heaven, but hopefully the book is accessible enough to attrect a wider audience. It certainly deserves it.
Kaku's approach is to look at the fictional invention, explain why it is impossible as it stands, but then go on to see how real physics could create something similar in the future. He classes inventions into type 1,2 and 3 impossibilities, possible in some form within the next century, possible in the distant future, and impossible given the laws of physics as they are currently understood. This is a framework which gives the author the opportunity to potter around on some of the more exciting playing fields of modern physics.
The most surprising thing about the book is the number of things he tags as type 1 impossibilities (starships, forcefields and teleportation amongst them) and the very small number of type three (perpetual motion, precognition).
The strength of the book is simply its source material. The whacky world of theoretical physics is one that should have interest to many beyond a purely scientific audience, especially when described in the largely layperson's terms used here.
My one slight niggle is that while Kaku is relatively easy to read, he isn't the most inspiring author in the world. His material is the inspiring part, and he puts it across well, but in the end I found the structure of the book rather repetitive.
Minor quibble though. Rcommended.