Unfortunately, the title oversells it slightly. Not all of these are "masterpieces"; some have been rescued from an obscurity that they richly deserved. Others are masterpieces, or at least good stories, but I had read about half of those already.
I got this volume (second-hand; no Kindle edition is available) the same week that the unfortunate death of its editor was announced, and thought that reading it would be a suitable way of honouring his contribution to the SFF field. It's disappointing, therefore, that I didn't think it was an especially great collection.
Some of the pieces are by famous pre-20th-century or early-20th-century authors, not all of whom are thought of primarily as SFF writers: Charles Dickens ("The Magic Fishbone"), Nathaniel Hawthorne ("Feathertop: A Moralized Legend"), Horace Walpole ("Hieroglyphic Tales"), John Collier ("Bird of Prey"), Frank R. Stockton ("The Bee-Man of Orn"), Mark Twain ("The Canvasser's Tale"), Saki ("Tobermory"), William Morris ("Lindenborg Pool"), Sara Coleridge ("Phantasmion"), L. Frank Baum ("American Fairy Tales"). Most of these, unfortunately, while of historical interest in some cases, are not outstanding or memorable stories. Some had not been reprinted for a long time before this volume, and generally I could see why.
Other pieces are genre classics from well-known SFF masters: Ursula K. Le Guin ("The Rule of Names", from the Earthsea setting), Joanna Russ ("The Man Who Could Not See Devils"), Gene Wolfe ("The Detective of Dreams"), Theodore Sturgeon ("The Silken-Swift"), Fritz Lieber ("Space-Time for Springers"), A. Merritt ("The Moon Pool"), Lord Dunsany ("The Sword of Welleran"), Poul Anderson ("Operation Afreet"). These stories are all deservedly well known, and found in plenty of other collections; I've read them all elsewhere, some more than once.
There's a third group of authors who are more obscure, and whose stories are also somewhat obscure; Margaret St. Clair's "The Goddess on the Street Corner", Wyman Guin's "The Root and the Ring", Lucy Clifford's "The New Mother", Christopher Pearse Cranch's "The Last of the Huggermuggers", Stephen Vincent Benet's "The King of the Cats", Kenneth Morris's "3 Tales". None of them inspired me to track down more work by these authors.
A fourth and final group contains well-known SFF authors represented by stories that I hadn't previously read: Elizabeth A. Lynn's "The Red Hawk", Anthony Boucher's "Mr. Lupescu", Ray Bradbury's "Uncle Einar", Avram Davidson's "Great Is Diana", Philip K. Dick's "The King of the Elves", Samuel R. Delaney's "The Tale of Dragons and Dreamers", L. Sprague de Camp and Fletcher Pratt's "The Green Magician", Robert A. Heinlein's "Our Fair City", and Michael Moorcock's apparently self-parodying "Elric at the End of Time". Most of these I enjoyed; a couple I found forgettable.
Averaging the stories out, then, I give the collection as a whole three stars. There's some good stuff, but some very ordinary stuff along with it, and about half of the good stuff is already widely collected.