Not my favorite computer science textbook. There was a great deal of mathematical notation and equations, presented with very little explanation; I'd characterize this book as a 'math-out'. The balance between theoretical and practical or technical content was highly biased in favor of theory and mathematics.
In addition, this book is fairly short and devotes relatively little space to any given topic. This is not necessarily a flaw - in many cases, the author is simply very efficient in explaining many topics, meaning that material can be covered with less reading. However, there are a number of topics in this book which I think the author elaborates on insufficiently. Since much of the book's content builds on previously discussed concepts, I find that the author's failure to provide significant detail or explanation on many subjects is a significant flaw.
This book does, however, have some positive qualities. Mortensen's writing style is clear and to the point, but peppered with an appropriate amount of humor and conversational tone. I appreciate Morgensen's decision to release the book for free as a PDF on his web site, as academic textbooks are often both very expensive and very large and heavy, and the PDF copy of this book is neither. Furthermore, the book is very nicely typeset (apparently using LaTeX), and the text is very readable. A large number of technical diagrams are furnished, and they are visually appealing and readable.
I think that this book is best used in conjunction with other texts and resources, or when supplemented by more technical or concrete instruction in class. The fact that it is freely available and relatively short are good qualities, but it does not provide an adequate amount of information on the practical and technical aspects of compiler design and implementation, and is often difficult to understand or parse.