It's been eight months since we first started hearing rumors that plastic Horus Heresy models were going to be released. A few months ago, leaked images showed a sprue of Mk IV Maximus armor and a sprue with two HQs: one in Mk IV armor and another in Cataphractii Terminator armor.
Additional photos have since confirmed that GW will be releasing Betrayal at Calth, a board game set during the Horus Heresy. Pictures of the back of the box show that the game includes multiple infantry models and a Dreadnought:
Although these models are supposed to be part of a board game, it's apparent that GW has much bigger plans than a single game. Space Hulk, which was obviously meant to be a one-off board game, had beautiful models but a minimal piece count and fixed weapons options
By contrast, the Mk IV Marine sprues are part of a full-blown multi-piece model kit with multiple wargear options (in one of the photos you can make out a plasma gun, a meltagun, a missile launcher, and several combi-weapon bits). From the look of them, the Mk IV parts are almost certainly compatible with other plastic power armor kits.
The Cataphractii Terminators are also from a multi-piece kit with a lot of options. It looks like there five pairs of lightning claws (I love the long-fingered Heresy-era lightning claws), five power fists, five chainfists, five combi-bolters, a heavy flamer, and a power sword.
The models on the HQ sprue have the least poseability (i.e., none) or wargear options (a plasma pistol and a power maul on the Mk IV model and a combi-melta and a chainfist on the Cataphractii model). I think most of us have gotten used to having limited options in our plastic HQs by now, and you could do worse than a Terminator Captain with a combi-melta and a chainfist.
Finally, the Contemptor Dreadnought is a little disappointing compared to the infantry sprues, although it's a step up from the usual starter kit Dreadnoughts. The pose is pretty static, but at least it gives you the option of arming it with a multi-melta or an assault cannon (Kheres pattern?).
(Most of these images come from War of Sigmar.)
It seems obvious that GW is trying to get more players into Horus Heresy. Personally, I'm still not interested in playing 30K, especially since the vehicles are likely to remain in resin (but give me a plastic Sicaran Battle Tank and I'll think about it).
Unless some of my fellow Cabal members decide to split the cost, I probably won't be buying Betrayal at Calth. I really want some Mk IV armor for my Tactical and Sternguard Squads, but I don't need nearly as many models as are included in this kit. If and when a 10 man kit of Mk IV Marines is released, I'll be the first in line.
As for the Cataphractii, I was on the fence until I saw the sprue. Now I'm willing to overlook the fact that I already have more Terminators than I can practically field in a standard game. I'm sure I could find a way to fit five Cataphractii Terminators into my army.