Okay, the first and foremost thing to bear in mind is that Susan Cooper penned these books way before JK Rowling started writing. So, if you notice the parallels of an English boy uncovering his magical powers on his eleventh birthday, well, you simply have to accept it and admit that Cooper did it first.
These books are truly AMAZING. I was attempting to figure out which one I liked the most, but they are all so excellent! They are much darker than the majority of children's fantasy. Extremely scary. When I was 10 years old, I couldn't sleep for days after reading "The Dark Is Rising". Also, you can read the first two books in a different order. Most people seem to read "The Dark Is Rising" before "Over Sea Under Stone". They will give the impression of being written by two distinct people, as "Over Sea Under Stone" is like a lighthearted children's adventure story, while "The Dark Is Rising" is much more, well, DARK.
Moreover, the stories are very closely linked to the legend of King Arthur, if that's what interests you. I won't say any more. You just have to read it for yourself!
I read the first two books of Susan Cooper's The Dark is Rising sequence approximately twenty years ago. What I can recall from that period is the contrast between those two books. The first one was a rather familiar holiday adventure story featuring three fairly standard adventure-age siblings. On the other hand, the second was a deeply atmospheric midwinter fantasy story set in a timeless English village where not much seemed to occur. In a sense, this contrast defines the series. There is the pull of the fairly standard adventures of the Drews siblings and the mysterious, atmospheric prose of Will Stanton's story. They alternate for the first four books and then come together in the fifth. I believe both were essential for my enjoyment. The quests that Simon, Jane, and Barney Drews undertake add impetus and action. Meanwhile, Will and later Bran exist in stunning landscapes and mystical surroundings but often seem to drift through that space without having much influence over their own actions.
I emphasize the atmosphere because these are some of the most atmospheric books I have ever read. The settings are always vivid, ranging from the fictional Cornish town of Trewissick, through Will's home in Buckinghamshire, to the mountains of Gwynedd which I am more acquainted with. These regions have remarkable weather, buildings, wildlife, and so forth, which contribute to a vivid impression of the world in which these stories take place - a world I inhabited, as it is Britain. I think that more than the Manichaean battle of the Light and the Dark, the atmosphere is what drew me back to these books and made them compelling to read.