If you've read Lev Grossman's wonderful Magicians trilogy you may find watching this series rather disorienting. The characters are all there and quite recognizable, and it's still about magic students and mysterious otherworlds, but it often feels like the series' writers cut the books into pieces, stirred those pieces in a bowl, then pulled them out and stitched the pieces together in random order, making up new stuff to fill in gaps.
That's not bad, and the world they have created works quite well, but it increasingly diverges from the books' mythology. This approach means I often find the show full of weird left turns where it shows me something familiar and then wanders off somewhere else with it, but the show is very entertaining, and it's prickly characters (awkward guy geek, ridiculously hot, short-skirted girl geek, snarky macho guy, snarky gay guy, angsty magic reject) are, if not quite likable, still fairly relatable.
The first changes from the book were relatively small. We follow Julia's story throughout the series, whereas her later reappearance in the book represented a plot twist. And while the books' Quentin did not have a Harry Potter-ish Destiny - he was just a smart guy so desperate for grand adventure that he would pursue all sorts of crazy things - comments made by supporting characters suggest that in the series, Quentin is in some way inherently Important. I feel that's a bit too obvious - the lack of destiny was one of those nice contrasts with Harry Potter - but it's fine.
Some story elements in the series are from the books, but they can come from any book in any order and are often mixed with things that aren't from the books at all. But while I'm often discombobulated by these changes, I'm rarely displeased.
Addendum: Just saw the last episode, so came back to say that this series held up throughout. Sometimes overly confusing, but overall very solid with an excellent finale.