The Custodes update has been pretty reliably rumored for a while now, but it's still astonishing to see the sheer scope of the release - it's actually faster to list the kits that weren't updated (the Pallas, Telemon, Aquilon Terminators and Gyrfalcon jetbike [I wouldn't count on the Orion getting a plastic kit any time soon]).
What's most interesting to me is the update to the basic Custodian Guard squad. These models were brand new for Burning of Prospero so aren't even that old, and those were widely considered to be faithful adaptations of the Visions of Heresy art at that time, but these new miniatures look like they leapt right off the page.
The Contemptors Galatus and Achillus are both home run adaptations of the Forge World kits and the inclusion of additional decorative parts akin to the Astartes Dreadnoughts is going to be a huge benefit to anyone that wants to run more than one. Likewise, the Caladius and Coronus both look to be pretty close to 1:1 to their respective resin kits.
I am not crazy about the changes to the Venatari, though. The simplified armor and overtly rocket-propelled flight pack (the originals looked more like Swooping Hawk wings and, I think, were meant to suggest a similar method of operation) is a miss for me, so much so that I wonder if these weren't originally intended for 40k and got pressganged into the Horus Heresy range.
And that's just the first wave of releases - we can still reasonably infer the remaining resin kits, a Sagittarii and alternate spear upgrades for the Custodian Guard squad, and the entire lineup of Sisters of Silence all still somewhere off in (the grim darkness of) the future.
And I still haven't given up hope that there'll be a Contemptor-Exemplar upgrade kit some day.