At time of writing, this 16-CD box set is available for a price of about £1.50 per disc. Considering it contains so many of the big hits and big hitters (both composers and performers) of 20th century concert music, that's absolutely staggering value for money. It may not be quite so appealing to connoisseurs who are already very familiar with the music, but for curious non-experts who may know some of the pieces but want to become better acquainted with a wider range, it's hard to think of a more comprehensive, and less expensive, way in.
Other reviewers have itemised the contents, so I won't repeat their work. They've also queried some of the selections. As a non-expert, I can't really comment on that (though I think the mix of the familiar and approachable with the more challenging is stimulating as well as well-judged), and no compilation is ever perfect, but to me, any serious quibbling seems mean-spirited given what's on offer here at this price. It does avoid the wilder shores of the post-war avant garde, but that probably makes it more accessible to a wider audience, and it's hard to see that as a bad thing.
If you're looking at this item and thinking it looks pretty good, well, it's outstandingly good, and you should order it without a moment's hesitation. You probably won't like everything, but you certainly won't regret it. Unreservedly recommended.