It's a pity this book hasn't reached a wider audience (judging from the few ratings and lack of any detailed review here, so far). It occupies a little niche of its own without being merely a novelty.
The book is great on many levels. Don't let the "irreverent" or "illustrations" parts in the title give the wrong idea. As a serious, straightforward history of philosophy, it's the most interesting and informative I've come across (compared to books of similar length of course, not multi-volume philosophy encyclopedias/histories). The author may have a cynical, negative bias on the value of philosophy (despite his philosophy background, Oxford degree, etc) but this becomes an asset here. A single volume on history over millennia inevitably has to have the ideas written in a short and down to earth manner, perhaps as unromantically or unlovingly as possible. The book covers most schools and philosophers that a layman may have heard of (and perhaps many more, depending on the person) and most sections are surprisingly deep. Despite the inevitable brevity, I often reread parts to follow the concepts/arguments, there are few words wasted.
On another level, the humorous parts aren't shallow attempts at crudity or light-heartedness, they almost always connect to the philosophy. I write the review a few years after I read the book, and what makes me remember more the serious parts is the creative analogies, the parodies, etc. The fun part was educational.
As for the author's arguments throughout the book (and especially in the end), he tries to show the wordiness, over-complication, pretension and dogma (as well as other negatives) that he finds making constant appearances in the history of philosophy. But he's self-aware to understand that his ideas could be classified as philosophy also, that his opinion isn't that unique (there's a rich history of anti-philosophy philosophers), and more importantly, that once philosophy bashing is taken too far it becomes fanaticism/dogma, what he criticizes throughout. And for another exception to the rule (the dogma), the section on David Hume is surprisingly so quiet and respectful that considering the general negative tone, it becomes equivalent to a moving, glorious tribute. I write this to try to show that even if one disagrees with the author's premise and arguments, the book can still be extremely enjoyable and engaging, you can take what you want from it, whether you take philosophy seriously or not.