I'm not sure what caused me to like this book so much in the first place, but I did. Maybe I was drawn in by the portrayal of magicians as fusty old historians. I stayed for the dry humor and the wonderful depiction of fairies (aka, the original one
However, I can easily see the complaints that the other reviews pointed out in the book. It's written in a sort of pseudo-period style, although it's nowhere near as difficult to read as actual works from the age. It's often difficult to see where the plot is going, and several of the threads drag on for perhaps longer than they should have. But still I found the book interesting, even through the first 300 pages that everyone says are a bore.
The last third of the book is very good, but it's really up to you as a reader to figure out if you can get there. It will require a long trek through 600 or so pages of what you might not find terribly interesting. I found the prospect of theoretical magicians rather intriguing in and of itself, which was enough to propel me through the first half of the book on its own. But sprinklings of actual magic do appear, and they are all entertaining and become more and more eerie as the book progresses. And by that point I had already been fascinated by the gentleman with the thistle-down hair and his wonderful faerie-ness.
As to claims that the book isn't funny... I just don't understand them. There were more than a handful of passages where I laughed out loud while I was reading. It's certainly a very dry humor, but it is there. There were even things that were blatantly silly (such as Mr Strange's belief that Venice was full of pineapples).
All in all, I enjoyed Jonathan Strange and Mr Norrell. But I think it would be very understandable if someone else did not.