It's good. It's very good. I found the magic depiction fascinating and original, and certain scene setups and plot twists are extremely creative (the entrance exams, the foxes, the south pole, the beast/watcherwoman twist) It's also very flawed, and has those flaws which really stuck themselves in my face.
FYI I'm going to spoil some of the events of this book like hell from here on out, so better run if you don't like that.
In no particular order, here's the flaws (or semi-maybe-flaws?) and why they bothered me.
1) I'm not sure if this is really a flaw or a feature or both, but at times the author slips so far into the Harry Potter pastiche (first half of the book) and the Narnia pastiche (second half of the book) that he practically seems to be trying to rewrite those books but really hastily and not as well, though with some excellent ideas of his own and a shiny new adult tone. (By the way, what was up with the Gulliver's Travels bit with the centaurs near the end? Sheesh. No, I'm not talking about Lilliput, but rather the horse people.)
2) The narrator is:
a) a moron (not necessarily a bad thing)
b) really depressing to read about at times because he is CONSTANTLY depressed and only rarely happy. Some really great moments in the story are undercut by his detachment & complete lack of caring.
c) extremely distant and alienated from everything, including the frakking reader. Good luck trying to get emotionally attached to this mopey fucker.
3) The characters have depth, but not enough. They're sorta broad types; they seemed realistic enough but I only ever cared about them or their fates in brief moments.
4) This may just be me, but for a book as straightforward and explicit as this, what's up with the sex scenes? They're super overdescribed yet barely explain what's going on. This is perfectly excusable (and probably the right choice) for the first sex scene (with the foxes), but did the book really have to tiptoe around that aspect and not everything else?
5) All the characters feel disposable, but the female characters seem especially disposable. Of the major female characters (and hell, minor female characters who aren't teachers), Julia gets screwed over early in the book only to be thrown in later as a Replacement Goldfish for Our Hero's Love Interest Slot, Amanda gets eaten (first book death IIRC), Janet is treated as a 'slut' and therefore disliked, and Alice is alternately fascinating then boring when the author doesn't know what to do with her - ultimately a pretty banal and annoying he-cheats-she-cheats storyline gives the author an excuse to keep her quiet for a while until she dramatically incinerates herself (but she might be back in the next book anyway, says the author). Finally, 11th hour badass Fen is the only casualty besides Alice of the Climactic Battle (both female).
On the other hand, the guys get pretty screwed too, and while the females seem disposable... so do the males! It's still a bit disproportionate, though.
5a) Speaking of which, the stupid cheating storyline. This is at least partly a personal peeve; as a polyamorous person my immediate reaction is (oversimplified) "why can't you just all have sex and be fine with it?" And then, I'm bothered by that old trope that tells us that people lose their worth when they break relationship rules, so long as it's sex. (Breaking other relationship rules just makes you a jerk. But sexually cheat, in a story, and your worth as a human being skyrockets downwards.) But even beyond that, couldn't it have been slightly more complex and less mundane of a story choice than it was?
6) The magic. This is both a huge benefit and a flaw of the book. What we get is overall pretty fascinating. But the author seriously skims over the basic details in places while casting others. What does it feel like to cast a spell? We only rarely know. We just get a vague description of mechanics and a cool description of the effect.
7) The narrative pace. Dear God, could the author have skimmed any more? This with a character that's hard to get attached to in the first place. We get oceans of time skimmed over. Additionally, the structure jumps around like mad, and it's a bit jarring - did we really need to get "surprise, Fillory is real!" like 2/3 through the book?
8) Confusing cliffhanger endings that leave the reader confused. Yes, this happened several times. And not to nitpick, but I wish some of the dialogue tags had better clarity, at certain points I'm unsure who's talking (and they all sound the same anyway).
...And I believe that closes out my first impressions.
All in all, a great book, if very flawed. I want to read the sequel, certainly.