Sometime back when I was in grade school, I picked up my older brother's copies of The Hobbit and the Lord of the Rings trilogy, and got forever hooked on fantasy. This was right around the same time that Susan Cooper's The Dark Is Rising sequence of books was published, but unfortunately (because I'm sure I would have have loved them then) I was oblivious, and only became aware of these books in the last few years.
My recent reading and appreciation of The Grey King was hampered by a couple of things. First of all, I have not read the other books in the series, and I'm guessing that you really need to have read Over Sea, Under Stone, and then The Dark Is Rising, and Greenwitch (the three books preceding The Grey King) to really get into Will Stanton's story.
Secondly, I just finished a couple of other fantasy books that were so very, very good that other fantasy just pales in comparison (Lois McMaster's The Sharing Knife: Horizon, and a re-read of Neil Gaiman's The Graveyard Book, in case you were wondering). Do you ever have this happen? That what you read influences your views so much on the very next book you pick up? Maybe I should have read some non-fiction next, or something else that really couldn't be compared.
Finally, I was stuck in bed recovering from a stomach bug when I read The Grey King. I will say Cooper's book helped me forget my surroundings for a while. But overall, I was a little disappointed. I didn't think that the fantastic parts of The Grey King meshed particularly well with the details of everyday life on a farm in Wales (at first, at least, I think it got better as the story went on), and I was confused about the relationships of the "Old Ones", the Dark, the Sleepers (pictured in the cover above), Merriman, and the Grey King.
The main character, Will Stanton, was an appealing character, and I liked his aunt's family and his friend Bran and Bran's dog Cafall very much, and the use of Celtic myth and history. But I don't have a strong urge to read the rest of The Dark Is Rising series - which I guess tells you something about my feelings about The Grey King. I don't have any problems with recommending it for young readers who enjoy fantasy, but for whatever reasons (and probably at least partially for the ones given above), it's not a keeper for me.