Top Grandmaster analysis, computer-checked, state-of-the-art variations makes NCO an excellent reference for stronger or very serious players. However, BE WARNED, NCO has little or no prose explanations of its evaluations(You've got to figure out why White is better, worse, etc.), neither are "thematic" ideas discussed. And NCO does not give those plausible or discredited lines that Kasparov, Karpov, and Anand would never play but are played by that smug fellow at your local club (So you're still on your own, unless he does you the favor of playing the "best" lines). Finally, unless you're familiar with ECO cataloging, the format isn't all that friendly (You've got to know what the Benko Gambit is to look it up, for example). I am only rated USCF 1940, and prefer more tutorial advice than NCO gives, and really prefer Modern Chess Openings Edition 14 by N. DeFirmian as it also gives one page intros to the major lines (as does NCO) but ALSO gives prose evaluations as to why White is worse, better, etc.. It is also VERY easy to look up openings, and its introductions make one curious to play out an opening. Take a look also at Standard Chess Openings by Schiller which is filled with prose desriptions, discussion, and analysis of the openings, as well as opening tips, repertoire possibilities. The drawback though is that other players claim it is far shallower than either NCO or MCO, as well as being less accurate. I have NCO and MCO, the first is for "status" and the second, for use.