Quick review for both HH box sets: In some ways they are great, on other ways they are lacking.
First off, the MkIII and MkIV armor kits are quite nice. The casts are clean and well put together and you get a good variety of parts. My only real disappointment is that they include a ton of little accessory bits (pistols, knives, 'nades, ammo packs) and I really doubt anybody putting these kits together would actually use all of it, and these gubbins take up a lot of sprue space that could have been used to add an additional special weapon or two. Take out a half dozen accessories and they could have included a grav gun and volkite charger/caliver. This is especially true for the MkIII which come with fewer guns because the legs are cast in two parts (necessary due to the higher complexity in the armor plates compared to MkIV). But on the whole these are good kits.
Next are the termies. I think both the cataphracti and tartaros kits are great. They go together well and have a ton of options for both tactical and lightning claw variants. Having put together AoBR, DV, and both the standard assault and tactical terminator kits I can say without a shadow of doubt that these kits are FAR superior to all the other versions in my collection. If I ever need more terminators in the future it'll be one of these two kits.
Now for the first big let-down. The contemptor dread kinda sucks. They stuffed the whole thing on a single sprue and the design suffers massively because of it. The whole thing is single-pose with the exception of the ball joints at the shoulders and even those will be limited to to a narrow range due to aesthetic limitations. A second sprue would have let them do so much more articulation and include more weapon options. This was a big missed opportunity.
For me, the biggest surprise was the Sisters and Custodes from Prospero. I didn't really like them all that much in photos, but holding the sprues in-hand changed my mind. What was going to be ebay-ed I'm now going to keep. The Custodes in particular look fantastic and I don't think GW's studio painters really did them justice.
Finally, the character models. The Calth characters have a big advantage in that they can be used for any faction. They look good, but are single-pose sculpts. The Prospero characters are unfortunatly puppy and sons specific. They're great models, but useless outside those factions.