The Sandman is a very special series which takes advantage of the comic book format to present something unique. It uses mythology and literature for its substance, and creates new icons that are almost indistinguishable from the "real" mythology. Reading Preludes and Nocturnes is a rewarding experience, and the more effort you put into understanding the layers of the story and the allusions in the narrative, the better the story becomes. It draws on the whole history of storytelling, from ancient myths and legends, via Macbeth, to Batman and Swamp Thing.
Preludes and Nocturnes is the first book of the series, and that is very evident. The series' creators hadn't yet found its sweet spot, and were still trying out new things and new ideas. The art is choppy and harsh, and sometimes difficult to decipher, especially in panels that contain a lot of action; but those qualities make for a very interesting visual experience, especially in the larger splash panels.
The book's flaws are mostly in its form, not its function, and they are growing pains that are soon dealt with. This is a book about ideas, archetypes and, most importantly, dreams. The Lord of Dreams has much more power than we might initially give him credit for. Not only does he decide whether we have good dreams or bad, but he controls the things we dream about while we're awake: our hopes; our ability to hope.
Dream himself is a character of surprising depth. He can be vicious and vindictive, but he can also be gentle and compassionate. He's surprisingly vulnerable, although one would have to know him intimately to be able to tell when he's hurt. He comes from inside us; in a sense we give him life by dreaming, but he doesn't think the way we do. We recognize him instantly, but we don't understand him for a long time. Gaiman presents a classic quest story, but he tells it in a way no one has seen before, with characters no one else could have created.
Preludes and Nocturnes has long been a mainstay of "Best Of" lists in comics, but it may be a bit daunting for a newcomer to the genre. For the same reasons, it is a fantastic book for more experienced readers.
Amazon: "By Gaiman's own admission there's a lot in this first collection that is awkward and ungainly--which is not to say there are not frequent moments of greatness here."
IGN comics: "...The Sandman is about the concept of dreams more so than the act of dreaming. A dream is more than just subconscious thought unleashed in a sleeping brain."
Stephen King (Powell's): "Neil Gaiman is, simply put, a treasure house of story, and we are lucky to have him in any medium."