Fantastic in both senses of the word. Though I would caution that you will need to enjoy at least three, and quite possibly all four of the following; long books, the world and machinations of early 19th century Britain, fantasy, and footnotes.
The footnotes alone are genuinely worthy of high praise, they wonderfully punctuate the text with all manner of off beat and inventive world building.
The text had me thinking like a 19th century gentleman for weeks, the text feels old. Brilliant conception and execution.
The fantasy and magic are both novel and have solid internal logic. The historical fiction is first rate (great cameo by the Duke of Wellington), the historical fantasy too (the history of the Raven King pervades and inspires the book to greater heights).
At times desperately sad, often completely hilarious. Usually when the silver haired gentleman is around.
Huge range of disparate characters - it takes quite some time to meet the first titular hero, the most stuffy Mr Norrell, and even longer to meet the more interesting but neglectful Jonathan Strange.
Personally loved Mr Honeyfoot and Childermas.
While everyone surely must love the wretched Vinculus!
It does feel long, a few hundred pages could have comfortably been dropped. Similarly, a few extra pages and it could have easily been split into several books, credit for not doing that, (possibly due to the lack of clear cut off points).
.