Amazon currently has this book for around $60. At this price, I consider it an easy buy because of the wealth of information. Particularly good are chapters 12 and 5, which are about the theory of constrained optimization and conjugate gradient descent respectively. I've spent time on and off trying to understand the Karush-Kuhn-Tucker conditions (which is essentially the fundamental theorem of constrained optimization) without much success. After reading Chapter 12, I wondered why I struggled to understand it at all. The writing is so clear and rigorous, and NW provides so many examples, that's it's almost impossible not to understand and appreciate the KKT conditions. As for Chapter 5, I never really studied the conjugate gradient method before, but I had no trouble understanding at least the basics after finishing the exercises from the chapter. Given that one of the most read papers on the method is titled "An Introduction to the Conjugate Gradient Method without the Agonizing Pain", and I had none implies that NW did a damn fine job with that chapter.
The book, however, is not without its faults. Some of the chapters, especially the ones on Interior Programming for Nonlinear Programming and the one on Derivative-Free Optimization, seem like throwaways. Also, I found a number of errata not yet reflected on the most recent errata sheet from Nocedal's website (although I may send a mail about the problems if I get around to it). Most of these were simple typos, but at least one was on a problem that made its solution impossible, and another was on a fairly important theorem in the appendix.
My biggest disappointment with the book, though, was that most of the exercises were not especially difficult or illuminating. I've attempted 90-95% of the theoretical problems and, thinking back, I can only think of two really great ones: one in which you have to prove the Kantorovich Inequality (which, although not posed as a probability problem, you can elegantly solve using expectations), and one - denoted hard in the book - on a minimax lower bound of a derivative-free method. I can't vouch for the programming problems, because my computer has all sorts of issues right now, but once my new one comes in, I'll probably take a look at those problems and provide an update.
These caveats aside, I highly recommend this book. I think the standard reference for this field is Bertsekas' "Nonlinear programming", but if "Numerical Optimization" is not already considered to be a must-have, it should be.