This is a classic example of a literary author tackling a genre for which he has no real love. The Magicians
is an adult version of Harry Potter meets Narnia. Unfortunately, "adult" in this case just means "remove all the joy, wonder, and heroism, and add sex, drugs, and swearing."
If this were a gritty, realistic take on the magical wizarding world, it would be a welcome addition to the urban fantasy genre, but it's "realistic" only in the sense that magic turns out to be arduous and rote, and the characters are all selfish, pathetic spoiled rich kids who are not improved by any of their magical experiences. It's almost as if Grossman wanted to stick a knife in the back of anyone who approached this book with a love of the material that inspired it.
For the first half of the book, there is no real plot, just following the main characters through their school years. Unfortunately, we don't see them grow much as a result -- indeed, there was no character growth at all throughout the novel.
The worldbuilding was okay, considering that it's just a blatant rip-off of Hogwarts set in upstate New York, with "Fillory" being a Narnia for infidels. The writing is decent and there were a few bits here and there that I liked, but for the most part, this book felt like one big sneer at fans of YA fantasy.