Middling as a primer, but great as a "spot check" for one's overall understanding of the game. This is not the first book a beginner should read. Opening knowledge is really only useful if you have the skill to convert whatever advantage you get from the opening into a win. That skill can be attained (and trained) by solving puzzles, playing vs real opponents, and studying games played between titled players.
Theory has changed since this book was first published. Titled players (with the help of engines) have found active and interesting tries in openings the author wrote off as "dull". But I found the variations and game fragments instructive. The author's emphasis on understanding over memorization is valid. Sadly, the content is uneven. Analysis of classical openings comprise the bulk of this book and sections about the more modern openings (like the Sicilian) are very, very short. Also, the moves are written in descriptive notation (example: 1. P-K4 instead of 1. e4), which modern readers might find frustrating to decipher.