I'm giving Jonathan Strange a 5 for the simply reason that I thoroughly enjoyed it all the way through, but I'd warn all readers to be more wary than usual of reviews (including this one). More than many books, this one I think will be a matter of true personal taste and experience will be your only truly accurate guide.
To begin with, Strange is often referred to as a "fantasy" novel, an "adult" Harry Potter (ignoring Potter's self-obvious claim to millions of "adult" readers). If you're expecting fantasy in the form of Harry Potter magic (though done by bigger people employing bigger words) or Lord of the Rings-like quests and elves, be advised neither is here. Fantastical might be a better genre-word here than "fantasy". There is certainly magic here, both human and faerie (very different forms), but when one of the major storylines is how magic has gradually disappeared from England and when one of the major characters has as his purpose the destruction (not Black Tower hordes of evil monsters destruction but economic, social, or legal destruction) of those who would become magician, as you might imagine there isn't a lot of magic going on, at least not for the first few hundred pages. Those looking for a lot of wand-waving or fireball-flinging would best look elsewhere.
One of the signs of the book's maturity is that one can't really generalize too much about the magic in it. Magic is almost invisible in the beginning and near-constant toward the end. It is scholarly, bookish and tedious and also vigorous, physical and exciting. It is human and Faerie and a melding of the two. It is all-powerful (Spain complains about the rearrangement of several of their country's geographic landmarks) and ineffective (you can see visions in water but they seldom are helpful). It is the subject of dry articles in academic journals and the cause of near-rumbles in local taverns. It is wonderfully complex and realistic. It is at times dazzlingly original (ships and sailors formed of rain, statues anguished over crimes committed beneath them), and handled often as if it is the most pedestrian, mundane aspect of daily life, except for the whole raise-the-dead, towering-shaft-of-neverending-night sort of thing.
As for other fantasy genre elements, there is no band of diverse creatures setting out on a quest to defeat some dark lord; no tall, shining elvish archers; no nomadic horse-loving tribes. If you want to find a Tolkien analogue, it isn't Lord of the Rings but Smith of Wooten Major, an often forgotten story about the collision of the human and faerie realms.
Strange is also referred to as a historical novel. It is set in early 19th century England, Wellington and other historical figures make their appearance, characters travel in carriages rather than cars. But the book's historical setting, like its magical element, is more pervasive than emphatic. It exists alongside the characters and story and serves them rather than being front and center as is true of so many historical novels. One is always aware of the historical setting, but I don't think anyone would come away from Strange with a truly enlarged understanding of the time period, as say one might from Lindsay Davis' mystery series set in ancient Rome, where specific foods and social rituals and forms of clothing etc. are constantly set before the reader. The setting is utterly believable, I'm sure meticulously researched, detailed and accurate, but it still doesn't feel like a "historical novel". Which from my view is a strength not a weakness.
My advice therefore is not to place your should I read or not bet on the book's labels. What should you know?
It's long. Very long. Longer than it seems according to the page count, since there are pages and pages of small-type footnotes throughout. Is it too long? I'm sure many people will think so. It takes its time in setting up story and character, leisurely is probably too fast-paced a description. It is far from compelling in the first few hundred pages in the sense of "must turn page to find out what happens". Personally, I found it compelling through character and style rather than plot. If you prefer plot, then prepare to be somewhat bored until the latter third where it moves along more speedily and in more traditional compelling manner.
It's discursive. Very discursive. It will wander away quite often and sometimes at great length from the major plot lines, interrupting with academic asides or summaries/analyses of old folktales, or snippets of poetry. Again, some will probably find this maddening, some will simply skip the footnotes entirely. I liked the discursive nature of the book and found the footnotes often as enjoyable as the main text, always tolerable, and only rarely annoying.
It is often beautifully written. it's one of those books where you'll pause over a line to reread it or let its effect linger a little while, whether due to the simple beauty of description, the efficiency of its brevity, or its dry wit. It is a true pleasure to read. Not to find out what happened. Simply to read.
It's often funny. It is at times frighteningly dark. It has at times a domestic feel and at other times a grand mythic feel, especially in connection to the Raven King, the mysterious magician-king of old North England who also ruled over a land of Faerie and allegedly another land bordering Hell who disappeared centuries ago with the promise of return. Whether that return is to be wished for or not is the core dispute between the two magicians of the title.
It is character driven. There are many wonderful scenes of "action", more so toward the latter half, but they tend to be understated while the book focuses more on character. Both Strange and Norrell are fully-fleshed out characters, totally believable in all their assets and flaws. We are given the time to know them and if Strange seems more appealing due to his more active role in the book, Norrell is no less accessible or recognizable for his minor jealousies and passiveness, though we may wish to deny the same traits in ourselves. The story of their meeting, their partnership, their sundering and what comes next is one of the major storylines and one of the more engaging, even if it happens mostly on the interior and despite the fact that the characters themselves are not particularly compelling by nature.
There is a lot more one could say about this book. It's a lengthy work and a dense work. One could discuss the conflict between wild faerie and civilized England--presented in shades of grey rather than black and white. The sharp social commentary. The distant narrative tone. But this review is edging close to the length of one of her footnotes, so I'll bring it to a close.
Try Jonathan Strange. If it doesn't draw you along, keep trying. If it still doesn't, try skimming footnotes, skimming pages, dipping in now and then to keep up on plot and catch one of those well-crafted lines, then come in for a landing and try again word by word. It isn't a rollercoaster ride on a summer Saturday . But it is a memorably gorgeous walk on a crisp autumn day, filled with slow sensual delight. Highly recommended.