OK, since people are shy about getting the discussion going, I'll take a stab at it.
Obligatory disclaimer: This is the kind of book that nobody seems to be writing any more, the kind of book I wish more people were writing. So in that way I am obviously biased in favor. On the other hand, I would have been all the more critical if I felt she had tried to write a nineteenth-century type novel and blown it entirely. In fact, if it had been a botched attempt I am sure I would have hated the book.
But I think Susanna Clarke accomplished something extraordinary in JS&MN. Certainly in terms of catching the style and the flavor of the period -- I don't think I've ever seen anyone try to do this and succeed so spectacularly before. I also appreciate the complexity of the plot, the "history" of magic, the depth she put into some of her characters, and the way that she combined fantasy and real historical events. All in all, I loved this book. It was a leisurely read, and I tend to be the kind of reader who races right on through to see what happens next and how it all turns out -- and then if I love the book, go back and reread some of the best parts -- but with JS&MN I was happy to slow down and savor the experience.
I was very happy with the way she handled the magic. She told us quite a bit without telling too much. I always like the magic in a book to remain a little mysterious, so that it retains a certain numinous quality. Once you start explaining it in ordinary terms ... as far as I am concerned it becomes just that, ordinary, and this where so many fantasy novels lose me. But while she doesn't make that mistake, she does perform a precarious sort of balancing act by keeping the magic mysterious while making most of her practitioners of English Magic so very, very mundane -- one might almost say relentlessly mundane.
The book does have its flaws. Mr. Norrell is not a sympathetic character. Nor is he, in the way of most anti-heros, a swashbuckling rogue, a disillusioned hero, or a spectacularly creepy character. He has a great talent for magic, but he himself is a dry, petty little man -- small in every sense of the word. Thank goodness Jonathan Strange finally comes along. Strange is not the most sympathetic character ever written either (he can be particularly annoying in his attitude to Mrs. Strange), but the contrast with Norrell does put him in a favorable light (perhaps more so than he actually deserves), and at least he is much more interesting. But even though I didn't love these characters, I found them thoroughly believable. One reason (I think) why the movements of the plot are so often less than dramatic is because the characters insist on acting like real people would under the same conditions, rather than produce a lot of melodramatic flourishes to help the author spice up the story.
There are, however, several characters that I found much more interesting and likeable than Strange or Norrell -- and I think it would have been a better book if we had been allowed to see more of them: Childermass, Lady Pole, Arabella Strange, and Stephen Black. I'm of two minds about whether we should see more of the Thistle-haired Gentleman; I found him fascinating, but like the magic perhaps he would have lost some of his sinister glamour if he had played a larger role.
And now for a bit of a spoiler in the way of a question (highlight to read):
Am I the only one who thought Childermass was going to turn out to be the Raven King? Until very near the end, when we began to find out more about him, I was waiting for a big revelation -- which of course never came, as I was completely mistaken.