It's fun and lighthearted. Using games as a medium to teach the language is something some people enjoy, and is a lot less dry than most programming books.
It avoids taking sides on the editor war, by just ignoring it altogether and teaching Lisp. This is refreshing compared to most books, which hit you with the Emacs cinderblock to the face right in the preface.
It also stays away from ASDF and Quicklisp and even the entire package system in general, which is a lot of extra complexity that can be overwhelming to beginners at first (though they'll eventually need to learn about this if they want to continue using Common Lisp).
The main issue people have with the book is that it uses CLISP-specific code in a few places. CLISP's last release was seven years ago, and because it hasn't really been maintained it's beginning to bitrot. Folks in #lisp generally don't recommend using CLISP these days, instead recommending actively-maintained implementations like SBCL, CCL, ECL, ABCL, etc.
Using CLISP-specific code does allow the book to sidestep the issue of the library ecosystem, which is a plus. But a revamped version (or even a collection of errata) that ported the CLISP bits to a maintained implementation would make it a lot easier to recommend these days.