Karen Horney is one of the greatest of the Post-Freudian psychoanalysts. Horney's concepts of the root causes of neurosis, a basic anxiety, a sense of lurking hypocrisy , an idealized self image, conflicting impulses, are ingeniously insightful and, I have found, correct.
Her paradigm is very useful for understanding human behavior; not only the behavior of those who would be diagnosed with a formal disorder but also of those whose personalities contain elements of neurosis not extensive enough for formal diagnosis. Understanding her concepts can also be valuable for those of us who would like to understand our own neuroses better.
If you want to understand psychoanalysis, read Freud, read Jung, but read Horney as well. Without her ideas, your understanding will be sorely incomplete. Remember, Jung and Freud can be integrated with Horney, synthesis brings greater understanding. Read, learn, and enjoy the benefits.