I am also a former student of Professor Peterson's, taught on the manuscript of this book, and it made such an impression on me that here I am, tracking it down three years later to reread. There are many significant positives to this book, as you can guess from the other reviews here. My main complaint is that the 400-odd pages could be vastly condensed and more tightly organized without weakening the thesis. When the subject matter is this dense, there is some argument for restating important points, but I do think the author sometimes errs on the side of excessive restatement.
Another area where the book could have been improved is in the use of more anthropological data to support its various hypotheses. An interesting follow-up read to Maps of Meaning is Wandering God by Morris Berman, which spends more effort tying the factual aspects of human and societal evolution to the way modern-day society is organized and the way people relate to the world around them. He also has some very strong opinions about comparative mythology a la Jung and Campbell.
Overall, Maps of Meaning is highly original, thought-provoking, and very well worth reading. Expect it to make a permanent mark on the way you see the world.