I was a Horus Heresy skeptic.
I always liked the basic story, but thought it best left to legend and the fog of the distant past. Having it somewhat unclear made that it was the distant past to the "current" 40K universe feel more real. When they announced they were going to do a series of novels about it and flesh it out, I was disappointed. I didn't want the myths broken down in detail, didn't want to know what primarch X had for breakfast right before Isstvan.
I read the first trilogy, and wasn't especially impressed. Over time I've read some more, with mixed results, but overall as the series has progressed I've become more impressed. Not only are a number of the stories themselves good, I've found I've actually liked some of the things they're doing. The sheer logistics and planning involved is staggering - decades of background built up haphazardly, and they're putting it together to make sense with itself, and foreshadow the 40K state of things. Various writers, doubtless coordinated, working towards a definable (thought still very far off) known end goal. Collaborative universes have been done before - things like Thieves' World, the 1632 universe and the like, but they have grown more organically, without a real known direction. Lots of Easter eggs for long-time fans, some doubtless tongue in cheek. Nice seeing throw-away characters from 40K like Kargoth Bloodspitter get some attention. Nice seeing the ironies that what the Word Bearers had originally been building up before their censure is effectively what the Imperium is in 40K. Nice seeing foreshadowing for various successor chapters. Nice seeing a lot of the tragedy, like just how far 40K Kharn is from 30K Kharn. Nice having various discussions of the warp from different perspectives.
For a lot of what I've read, I mostly followed some authors, such as Dan Abnett and Aaron Dembski-Bowden, and branched out a bit for stories or legions I was interested in. So far I've read:
Initial trilogy - as said, OK but not terribly impressed. Might be worth a second read now.
Legion - apparently you either love or hate this book. I'm in both camps. As a "40K" book, I really didn't like it. The Cabal I could do without and the Perpetuals I could stand to see less of in the Heresy. Some separate series featuring the Perpetuals from pre-unification through the millennia I could dig, but in the Heresy I think they distract from the primarchs and legions - too many different types of being exceptional. But as a sci-fi book, I think it was very good. File off the serial numbers and it could have been a pretty interesting standalone book.
The First Heretic - Similar to Legion, I think I would have liked it better as a standalone. In a lot of ways it reminded me of Roger Zelazny especially Lord of Light, which is a very positive comparison for me. I felt some of it was a bit contrived, with everything being laid out ahead of time thousands of years in advance by the Chaos gods. If Cadia had been downplayed a bit I wouldn't have minded so much, but it felt a little heavy handed.
Age of Darkness anthology - I don't remember much of most of the stories, but as an Iron Warriors fan I of course liked the Iron Warriors story.
Know No Fear - Pretty rip-roaring once it gets going. One of the times I would have liked some more detail on some of the engagements and see some more with some of the characters that were built up.
Angel Exterminatus - I'm an Iron Warriors fan, so this was an easy sell for me. Nice seeing Perturabo as a frustrated builder, adding to the tragedy of his story as he could have really shined in a galaxy where the Heresy didn't happen and humanity stayed united, or in a post-Heresy Imperium had he stayed loyal as one of the literal architects of the new order.
Betrayer - Good discussion of Chaos and the warp, with Argel Tal getting some good lines, lamenting the nature of Chaos but saying that's the reality and it's foolish to deny it. Surprising hearing it from a possessed marine, but more effective as a result. Another aspect of the warp as a tool, an inconvenient reality that once you know it exists, you may as well use it. Contrasts with the fervent believers like Erebus, whose enlightenment can be as blinding as ignorance. Really shows what a jerk Angron was, and the tragedy of the War Hounds/World Eaters. Nice addressing the World Eaters' librarians.
The Unremembered Empire - Some potential 40K foreshadowing and showing unintended consequences (lighting up Macragge as a beacon...what's later drawn to Macragge for some reason?). I thought the basic idea could have been fascinating, showing Guilliman's genius for organization, showing him rallying Ultramar against the incursions and reordering his shattered realm, doubtless with some difficulty and interference. Instead it seemed too busy. By the end we have five primarchs involved, which distracts from Guilliman and in my opinion is just too much for the premise in my opinion. Add in Perpetuals and Space Wolves and so on and it gets too cluttered, like the third movie in a superhero movie series where they feel they have to keep tacking on extra heroes and villains.
Scars - I'm a White Scars fan, so another easy sell for me. More good discussion of the warp, with the discussion of the Librarius, how the stormseers use the warp but don't delve too deeply, and the great line of how even though the warp may talk to you, you don't have to obey it. Nice contrast with views like Argel Tal in Betrayer and of course other traitors. Ironic, or perhaps appropriate how the primarch with the most emphasis on speed is perhaps also the most patient. For him speed is not to be used to rush ahead, but to allow you to choose the most appropriate time to act, to be able to get more information and being able to act more successfully when you do. Some good exploration of the difficulties with Terran recruits and those recruited from the primarch's world, something explored as well in Betrayer with the pre-primarch legion and friction with how the primarch changed it.
So, I'm a convert. I won't read them all as they come out, but I will continue to read some of them as they interest me. When it's all done, I do think that even with some weaker books overall it will end up as one of the great sci-fi epics, and I envy anyone who will get the chance to read through it without exposure to the 40K storyline/knowing the outline of the Heresy already. I fear that most who start and get interested will end up exposed to tons of spoilers, but for the few who manage to get through it all unsullied, wow. I'm also quite pleased with how the success has helped lead to the Forge World Horus Heresy books, models and whatnot.