This story is full of well rounded characters, earth-shattering perils, and a laundry list of fantasy tropes. TL/DR: If you like High Fantasy (as opposed to the gritty stuff currently popular) then read this and be pleased.
Tolkien is visibly present in this book. He is there in the "mortal races band together to fight evil god they already beat once-also, elves are better at everything, and there is a proud race of horse riders on the plain", and "lets have an prolog that tells a thousand years of history". Lewis is there in the "young people from our world turn out to be important in a fantasy world". There are also snips and snails from various European myths and folktales, justified in canon since Fionavar is the the "first of all worlds", so our tales are just echoes of the originals. There is an Action Girl, a High King, a Lovable Rogue, and more.
Kay has said that the book is supposed to be derivative, since he wanted to see how much emotional and moral depth he could explore within the constraints of High Fantasy, but for me the similarities were jarring. Also, the use of overly pompous syntax got on my nerves. In-book, that would have read 'On my nerves, did grow the use of overly pompous syntax'. Thankfully, it was mostly present in the only a few plot lines, those taking place in the High Kingdom (because of course there's a High King), and when the plot is on the plains with the horse-riders the writing is much more naturalistic.
The characters are this book's saving grace. Kay is a master at breathing life into his characters, giving them both noble traits and flaws, deeply held beliefs and contradictions held just as strong. If a figure comes across as one note, it's usually because we just haven't gotten to the reveal yet. There is emotional depth to the characters, and an exploration of morality, of courage and choice, and duty. One character is wracked with guilt, and must struggle to overcome it, Another is filled with shame and anger, none of them are what they seem at first introduction. They must all deal with what they learn while in the other world. I just wish the heroes, college students from our world, weren't so good at doing the fantasy stuff right away, especially the city kid who is suddenly a great warrior on horseback.
Just beware that this is Book 1 of a trilogy (Fionavar Tapestry), and forgive the fact that some characters just seem to be introduced and then do nothing, and that it ends on a non-ending. It was never meant to be a truly stand-alone work.
If you love this book and want to discuss Kay's decision to create a derivative work, join Eric and the other members of the Out of This World book club as they discuss a new science fiction or fantasy title each month.