The novel is a book about discovery, such as when Quentin discovers he enjoys teaching and research back at Blakebills. But the plot is extremely contorted, and more than a little silly, with little twists that turn out to be "uh, why did you bother" when the big reveal shows up.
Quentin as a character finally comes into his own in this novel. Fantasy novels do end up with power creep in a big way, as characters get more and more powerful in order to scale up to more and more challenging threats. The Magician's Land does the same, with Quentin becoming more and more powerful until the climax of the novel, where he literally becomes a god. The neat thing about it is that you do see him actively work on it, so it's not as bad as I've seen in other novels where the characters get vested with power.
The first third of the novel is a magical heist caper. It's fun to read, but doesn't quite fit in. Furthermore, one of the characters who's a big player in that section sticks around for the rest of the adventure but never amounts to much other than window dressing. The rest of the novel revolves around the end of Fillory, which resembles Narnia's ending far too much though thankfully does away with Christian allegories.
What bugs me about the novel is that it had a natural ending, with Quentin sealing his ties to his beloved Fillory forever, but instead veers off in order to leave things open for a sequel. The resulting end of the novel doesn't feel as solid as I thought it could have been, and smacks too much of commercialism.
It's a reasonable read on an airplane, but I wouldn't go out of my way to find this novel to read or pay full price.