More than a double biography, it's a journey from the dawn of video games to the advent of 3D gaming.
A little romanced (of course), but still fresh and relevant.

This book is about the rising of FPS, how and why it touched so many people (especially north-americans), and the men behind it. The fact that the author followed the two main protagonists for several years, especially when they finally splitted from each other, is important to the relevance of the whole story. We really have two points of view on the events, even more when the other members get their share of limelight.

It's also about two approaches about game design : the constraints (Camarck) and the possibilities (Romero).
Either you build the technology then try to use it to its best, or you design the best/fun uses then build the technology to implement them. Through the two johns, we see the good and the bad of those opposed philosophies, and how they work the best when reunited.

We are also treated with the inner cycle of game creation, where several no-life guys sit in front of a computer for months day and night, the internal/external complexities of business and, of course, the human factor. Quite frightening, considering it was before the rise of AAA games, at a time games weren't as complex and demanding as they are today.

The only real flaw of the book is its age.
The story told here stops a little after Quake 3 Arena. 2003 is quite old in the video game world, and lots of thing have changed since. While still in the business, Carmack has pursued his new obsession with space travel, winning the X-prize. Romero continue to explore game designing in the mobile market. In that regard, an updated epilogue would have been a nice addition.

On a side note, it's funny to see that, from all the contenders of the 2000 era, only Half Life (the underdog at that time) managed to stay relevant to this day. A game with no multiplayer ("PvP is the most fun you can get from any game" - Romero) and a big emphasis on story ("video games are like porn movies: story is irrelevant" - Carmack) ended up being probably the best example of "design + technology".