Rounding up to 5 stars because this book takes a lot of flak from people who just aren't the right readers for the book.
Jonathan Strange & Mr Norrell is the story of two magicians in early 19th century England.... an alternate England, readers quickly discover, where magic is widely accepted (if no longer practiced) as a part of life. The strengths of this book lie primarily in the character development, the author's excellent recreation of early-19th-century-style prose, and the superbly drawn backdrop and atmosphere behind it all: from fashionable London social gatherings to the bleak northern England landscape where the heart of English magic seems to lie.
If the above paragraph doesn't constitute much of a recommendation in your mind--in particular, if you're distressed that I did not say anything about a fast-paced plot or action scenes--then you should not read this book.
I was immediately drawn in by the first scene of this book: a rather humorous account of a meeting of the Learned Society of York Magicians, none of whom have ever done any magic; within a couple of pages I understood why Clarke is often compared to Jane Austen and Charles Dickens. The characters come instantly to life, even the ones who aren't very likeable. The relationship between the title characters is the focus of this novel, and it's so complicated and human and believable that I couldn't help but keep reading.
The prose is great and does an excellent job of maintaining the sense of immersion in the time period. Most historical fiction authors make no attempt to use period language--nor should they--but Clarke can pull it off and it's brilliant. I love her subtle wit, and the footnotes are excellent. Normally I'm not a big fan of footnotes, but the way Clarke uses them, to give us background information, flesh out side-stories and cite to fictitious works of magic, adds so much to the story.
And of course, the alternate-history worldbuilding is excellently done. Clarke does a great job re-imagining how England would be with magic as part of its makeup; quite similar to the real England in many respects, but the deviations are well woven into the setting and learning more about how it all fit together kept me reading. And as I mentioned before, the atmosphere and scene-setting are excellent.
I will still voice a few gripes. Some of the secondary characters did not seem quite as well-developed as they could be given the amount of page time they got; I suppose it's their English reserve getting in the way, but I didn't feel that I knew characters like Childermass or Lascelles very well at all. The scenes with Stephen Black--although he's a great character--became repetitive. And especially for a female author, I found Clarke's female characters a bit passive. They have personality, but when a man and a woman wind up in the same unpleasant situation for most of the book, he manages to carry on as normal while she does little but sulk. For a decade. Come on, have some strength of character, please.
At any rate, I highly recommend this book, but to a specific subset of the population: only if you're happy to read a long, slow-paced book for the characterization, setting and prose. Read the Amazon excerpt before you buy.