Dr. Karp is part of a new movement of parents who prefer gentle approaches to raising children, and the data is on his side. Not only do his techniques work, but everything we know about children tells us that straying from these sorts of gentle approaches causes psychological damage. Karp is buiding on the work of Dr. Lawrence Kelemen (To Kindle a Soul: Ancient Wisdom for Modern Parents and Teachers), and he does a good job. Kelemen's book (To Kindle a Soul) is still the classic guide to getting great kids using gentle techniques. I strongly recommend Karp's book for those interested in putting their children to sleep, and I strongly recommend Kelemen's book for gentle and incredibly effective techniques to handle misbehavior, discipline and punishment problems. Kelemen's book also has a lot of information about gently handling ADD and ADHD kids without medication.
I don't like the way it was written. I find the "scientific" explanations to be speculative at best. The "best" part of the book could be condensed into a much, much shorter volume. Maybe people don't feel they'll get their $10.20's worth if it's only 50 pages?
Despite all that, the trick that it teaches (how to sooth a crying infant, 0-3 months old with swaddling, white noise, side/stomach lying, jiggling, and non-nutritive sucking) absolutely worked for my baby. And it worked quickly, completely and every time. My advice....Skip to the chapters that explain each of the five s methods and those that concentrate on extending sleep, and you will cut out some of the fluff while retaining all of the most precious content.