Whilst awaiting a delayed train this morning I finally finished the Horus Heresy. That's a whopping 54 books (I know the series morphs into The Siege of Terra at this point, but I need a break to read other things so that'd where I'm stopping)
I thought this would be a fun challenge at the start of lockdown last March to read them all - obviously I failed at getting them all done during lockdown, but I could have got a lot closer had I not stopped for a few breaks due to it becoming such a relentless slog, at one point pausing to read six Harry Harrison books. In fairness the series isn't meant to be read back to back, but it does highlight the flaws in the series - overlong, stuffed with bland characters and needless descriptions of fighting and some very dubious plotting. The main failing is simply the quality of writing: of the 54 books I'd say about a third were good, a third average and the remainder a mix of lacklustre to outright dreadful.
That said the 40k lore is really good, and the overall arc is a good one. The characters of the Primarchs have this pseudo-Shakesperean setup where they're super awesome but possessed of a fatal flaw that brings them down, giving the series as a whole the feel of tragedy, just one a bit ineptly handled. In the downside, almost everyone is the series is a truly terrible person, so it's pretty unrelenting stuff.
I'd liken the experience to eating one of those man vs food style meal challenges where you get a certificate or a prize at the end: at the start it looks fun and full of delicious stuff you enjoy, but the reality of it is one monster slog of unenjoyment and at the end you feel tired and bloated and sluggish from it all.
Anyway, I'm off to read something that doesn't have any chainswords in it.