Having read the synopsis, I knew this book could potentially be quite sad. I don't do sad. Not in books, not in movies, not in any medium. However, the premise was so intriguing I had to give it a chance anyway. (I've somehow avoided seeing any of the cinematic versions.) I'm really glad I did. It did turn out to be sad, but totally worth it. Something I especially enjoyed was the accuracy with which the psychology was presented. Since this book came out in the 60s, I had guessed that it might espouse full on behaviorism, or worse, psychoanalysis. After all, I still see Freudian terminology bandied about today, as if the approach wasn't decrepit and discredited. It's not an uncommon theme in fiction, either. Fortunately, it turns out Daniel Keyes is a psychologist himself.
For my obligatory nitpick, I'll choose to point out that a few techniques in the book actually have been scrapped today. The author's masterful approach here leaves me nothing but these technicalities to criticize. Everything is spot on for it's time, of course, but a few things haven't aged well (IE the Rorschach test).