I've read the chapters available online and I've read the source code to all the chapters. It wasn't a long read. Some tricks were nice, others were well-known, others were not so good. Overall, it's not a bad book, but you should be wary that not all code in the book is idiomatic CL (things like omitting earplugs on specials, while using something akin to a hungarian-notation(albeit better since it's automatic) for gensyms and once-only's)).
If you're confident in your CL, have read On Lisp and PAIP, and want to learn a few new tricks, sure, read it!