Simplistically (and not very originally) put -- Harry Potter for grown-ups. I was always a little irritated at how little effort JK Rowling seemed to put into the process by which Harry & co. learned magic - say a few words (pronounced correctly, of course), "swish and flick" and there, you've done magic - which seemed to imply that the magic is in the words and the wand, not so much in the wizard. Grossman's descriptions of the training Quentin and his fellow students undergo at Brakebills makes clear that the bulk of the training is in honing the minds of these magicians-to-be, and everything there is geared toward breaking them free of old thought patterns, or stripping them of external resources to force them to tap their internal ones.
Despite the unconventional subject matter, the trappings of college life are still familiar - trying to figure out who you really are, developing new friendships, drinking, having inappropriate relationships... discovering that you are good at something, as one-time malcontent Quentin does.
Just as Quentin is getting ready to take up a career, he discovers that the land of Fillory, which he assumed existed only in his favorite childhood books, is real, and he and his closest friends embark on a quest to find the way to Fillory and learn its secrets. I was disappointed by the ending - rather than "aha!," and a sense that the resolution fit the lead-up, it was more "oh" and a feeling of dissatisfaction. The big reveal was a bore - less exciting than when the Wizard of Oz is exposed behind his curtain.
Looking forward to the sequel, which I've been told is even better than the original.