Well, that was quite a ride.
Having recently received OHC4, but having forgotten much of what went before, I spent the last few hours re-reading OHC1-3 then the finale.
In the end what makes it work is the central mystery and its resolution, which is both very effective and also been teased to the reader across the 40-odd preceding issues, albeit with much misdirection.
With a couple of exceptions (Holt, both old cunt and bitch daughter, and Cale) the cast of characters are very well developed and complex. Do they have their arsehole aspects? Oh yes, but those also get balanced out by their more positive traits. It's quite the tightrope walk that Seeley's pulled off here.
At the same time, Norton has supplied 47 issues of excellent art, depicting all manner of horrific imagery, but also needing to nail the emotional sequences that come up from time and time and which, arguably, the crux of the story revolves around.
The final smart move by both creators is knowing when to call it a day. These 47 issues form a very strong arc, with little to zero baggage or waste. In a lot of ways it's easier for a story to start and then somehow be kept going, by character resets or new twists, then be brought to an end. Seeley and Norton managed to nail it at the finishing line.
Whether you read it as 8 paperbacks or 4 big hardbacks, give this a go. Not a horror fan? I can't say I'm a big one of that genre either and yes, it has elements of that, but it also has a whole lot more going on.