I've been spending some time, off and on over the past several days, browsing through the pages on this web site. It's quite a nice overview of fantasy and science-fiction, all told, although inevitably I have points of disagreement. Obviously, I'd argue with the ratings of some of the writers; I think John Crowley is undervalued, for example, and Terry Pratchett significantly overvalued. I also find a few writers, like Alan Moore, Peter Ackroyd, Elizabeth Hand, Samuel Delany, and Clark Ashton Smith, notable by their absence.
Also, while in general I appreciate the critical commentary and discussion of the criteria for inclusion of an author on the site, in practice those criteria are still based on personal opinion; is Jack Vance, for example, really worth a five-star rating? Some people would say so. I, with admittedly less of an exposure to Vance's writings, wouldn't be willing to go so far, and I suspect I'm not alone in that. In other words, this site, like all good literary criticism, is the beginning of a discussion and not the end of it.
Mind you, the discussion itself is worth having. It's invigorating. Good criticism makes you think about what you read and what you like and why; good positive criticism makes you feel what's valuable in what you read and what you like, and therefore may be of a higher order of writing. To see what I mean, take a look at the appreciation for John Bellairs' excellent book The Face in the Frost. Obviously, if you've read the book, this essay reminds you what's worth while in it; if you haven't read the book, then the essay stands (I think) a good chance of giving you the desire to track it down. Either way, the energy of the piece communicates itself. It tells you that the book matters, and so it reminds you that fantasy matters. That writing matters.
If nothing else, the site puts together in one place a rigorously-chosen list of writers worth investigating. And it's a list which extends well into the past. The paradox of science-fiction and fantasy is that sf&f readers are often passionate about their favourite contemporary writers, but almost oblivious of previous generations. There are exceptions. But while everybody knows J.R.R. Tolkien and C.S. Lewis, I suspect very few people have heard of, much less read, their friend Charles Williams. Or inspirations like George MacDonald, E.R. Eddison, and William Morris.
To gather all these writers together is worth something in itself. Like John Clute and John Grant's Encyclopedia of Fantasy, the Great Science-Fiction & Fantasy Works site provides inspiration and direction. It gives you names to keep in mind as you scan shelves in a second-hand book store; but it also keeps those names alive and associates them with their present-day successors. I'd argue that the real literary tradition of the 20th century is the semi-underground tradition of fantasy writing, as opposed to the above-ground tradition of modernism; spend some time with this site, and maybe you'll see why.
(And if not, well, it's just possible I'll write a post about it sometime.)