"Classic!"
I was young when I first read these books so I was excited when I found out they were finally available on Audible.com in the US. If you haven't read them in a while, LISTEN! If you've never read them, LISTEN!
"Exercises the imagination"
This book is full of concepts that made my imagination run wild. At times they were beyond my grasp. As he mentions in his foreword, he introduces ideas at a fairly understandable level and as he continues he builds to a point at which he will probably lose most readers. Personally I love this and plan to listen again after familiarizing myself more with the concepts.
"Definitely an interesting listen"
If you're interested in astrophysics and/or knowing what is out there this is a good place to start.