You may be looking for instructors' opinions on this book, not students', but for what it's worth, as a new skier coming into the sport late in life, I found this book tremendously helpful.
Before reading Elling's book I had taken a few group and private lessons and found various instructors in those lessons emphasizing different concepts so I did not think I was getting a continuous progression. Certainly the lessons have their place, but I got much more out of lessons, and certainly much more out of free skiing, after reading the Elling book. Now, I often re-read chapters in the day or so before a chance to ski, to provide focus for the next day on the hill. The book provides continuity and an overall framework that was lacking in lessons at different hills with different instructors over the course of a few seasons. It was very helpful in understanding the basics - what Elling calls the skiers' "toolbox." There's a lot that I still can't ski (bumps, bumps, bumps and heavy crud). But with the Elling book, I think I understand what I CAN'T ski, which hopefully will help in eventually mastering the remaining challenges.
I also read the Harald Harb book and it was not nearly as useful. Harb's book seemed very "drill" oriented, and I couldn't take that to the hill in my head. Elling's book was much more "concept" oriented; even if I forget the particular drills he suggests, I can bring the concepts with me (outside ski dominance, steering, stance, etc.).
That said, I'm very interested in what instructors on this forum think of the book. Please don't tell me that Elling has it all wrong, and really I can't ski after all!