This book has been described as Harry Potter for grownups, and I think that's a pretty good descriptor.
The Magicians is about Quentin, an apathetic, lonely teenager growing up in New York City. One day, after finding an unpublished manuscript by his favorite author (who is famous for a Narnia-style series of books) in the home of the man with whom he is supposed to have a college interview, he stumbles through a park and into the world of Brakebills, a school of magic. After several rigorous tests, he is accepted as a student, and the rest of the novel follows his time in the wizarding world.
I liked this book a lot--it's definitely a book for grownups. There is plenty of sex and violence, and Quentin is a sarcastic, often unlikable narrator. Grossman does a good job of setting his world apart from that of Hogwarts, as there are no cutesy owls or house elves in this book. The Magicians is considerably darker, taking the more depressing tone of the later Harry Potter books and magnifying it. Magic is dangerous, often uncontrollable, and decidedly scary in this world, and the potential for evil is much greater.
My biggest criticism of the book is that the Brakebills years, which only take up about half of the novel, feel sort of rushed. We fly through four years that could have easily been expanded into their own novels. I get why Grossman did this, to separate himself from J.K. Rowling, but there were times when it felt like he was merely glossing the surface of a potentially rich narrative. I do like how Grossman takes us outside of Brakebills in the second half, letting Quentin and his friends put their magic to use and understanding the true power of their gifts.
I really recommend this to anyone, whether or not fantasy is your thing. This is pretty far from the tamed-down world of Harry Potter, as good as they may be, and it's cool to see this type of book through the perspective of an adult.