Well, as I have been whining about for the past 3 weeks... this book was looong. And, apparently, I cannot stop reading a book. I guess I kept at it because I kept wanting it to "get good." And things did get going in the last 100 pages. But... that's a lot to slog through before getting to the good stuff.
I have just read a bunch of reviews to see what others have to say and people seem divided between "I love this book! [because I love Dickens, Austen, the Victorians, etc.]" and "This book was too long." Obviously, I side with the latter camp.
The story was interesting but could have been even more so: the basic premise is that these two magicians start off as allies (Strange is Norrell's student), disagree about the study of magic, and split ways, each to his own magical pursuit. But there was zero bite in this fight: They weren't at odds so much as they just wandered away from one another. In the end, when they have to join forces again, it's like, "Oh, yeah. We're back together." (yawn) and then... what they intended to do together was actually done for them by someone else, but they don't even realize it. What?!
It just felt like there could have been waaaay more excitement infused into the writing of this story: there are fairies and enchantments, places get moved around, people are raised from the dead, etc. That's some good meat! But it's overcooked and slogging through the surrounding... er... potatoes (?) was wearing.
Oh, yeah. And there was a crapload of endnotes, which I stopped reading almost immediately. I guess, if I was into the book and the style, I would find the fake endnotes (referencing text that does not actually exist) charming. And might even enjoy the misspellings of "chuse" for "choose," "surprize" for "surprise," and... weirdly, "shew" for "show" (the first two examples sound the same, but that last one is bizarre... maybe with an English accent...? I don't know.) But instead, it was just more work.
In my review reviewing, I did see some of that "Harry Potter for adults" comparison that you mentioned hearing when this book came out and I'm gonna say... No. Both involve magic, but that's where the similarity ends. In fact, I would love some more HP infusion in this book (and there were opportunities for it — several individuals sign up to be Strange's students and there is even a failed attempt at a School for Magic — but none of that gains any traction. Of course.) but the story is not nearly as exciting as an HP story!
I guess, if a person is "into" this sort of prose, this book would be good for them. I am not. Sometimes, the slowness reminded me of Tess (set in the same enormous time period generally classified as "Victorian") but I honestly thought the story of Tess (while also slow) was far more engaging and far more beautiful. This book left me feeling like, "That's it?" Which is, of course, a big bummer.
The good news is that I pretty much burned through it (I'm not nearly as fast of a reader as you are. Covering 846 pages of a book I'm not that interested in in 3 weeks is speedy for me!) I will also admit that the book might have suffered because I listened to two other completely engrossing audiobooks this month while reading it (The Fault in Our Stars and Gone Girl), so I did wish that it was more like those (that is to say: holding my interest...)
Oh, yeah! I also highlighted (highlit?) a couple of interesting quotes, so this isn't a total bag-athon:
From Norrell to Strange, as he takes him on as a pupil — I love this commentary on England: "You must argue and publish and practise your magic and you must learn to live as I do — in the face of constant criticism, opposition and censure. That, sir, is the English way.” 
Insightful observation on the problem of having two magicians in England:
“I mean that two of any thing is a most uncomfortable number. One may do as he pleases. Six may get along well enough. But two must always struggle for mastery. Two must always watch each other. The eyes of all the world will be on two, uncertain which of them to follow." 
And this made me laugh right out loud, as I have both known travelers and been a traveler myself — so true! “Oh! I shall not spare you. It is the right of a traveller to vent their frustration at every minor inconvenience by writing of it to their friends. Expect long descriptions of everything.” 
Overall, a good idea for a story, but way too slow. My next book is going to have to be a page-turner (sorry for the pressure, next book!)