I don't play much lately - I stopped going to the stores due to covid, and since then have started going to a new, closer hobby store that opened which currently lacks an active AoS player base. Still, I at least collect and actively follow news for Ossiarch Bonereapers (my new favorites) and Soulblight Gravelords (the familiar fallback). I've got handfuls of Flesh Eaters and Nighthaunts, plus a fairly large Tomb Kings collection - all hold outs of my oldhammer Undead Legion, but I don't really think about or aspire to play them in AoS any time soon. I'd like to bulk out the FEC and Nighthaunts, but I promised myself I'd get playable armies of the two main factions painted before I start entire new armies. As for the Tomb Kings, they're mostly still on old square bases, and I currently have them in storage with my fingers crossed that they'll become playable again in the Old World game, whenever that comes out. We'll see.
In terms of why I like them - the Ossiarchs have this cool, high fantasy, apocalyptic, necro-monger thing going, an active undead empire with their own unique culture and ambitions. That's just not something you get from typical fantasy undead, where whatever identity they have is always secondary to whowever they were before death. The ossiarchs also have a fun, toyetic goofiness to them, like 80s toy cartoon villains, which I know many don't like but is exactly up my alley. My only real complaint about the existing models is that the morteks are a bit short to be the super-elite warriors they're supposed to be in the fluff. I would have preferred them scaled up to stand eye-to-eye with their stormcast and chaos warrior rivals, and mounted on 32mm bases, even if honestly that would have made them less effective in game and probably would have resulted in them costing twice as much real world money per model. But short kings aside, I love the faction, and hope it gets some significant expansions soon (top of my wish list: archers, infantry lord, warsphynx-equivalent, chariot equivalents).
The Soulblight Gravelords on the other hand are the classic fantasy undead archetype, and, while I love the Ossiarch's for being a refreshing change of pace from it, there's a reason why the classics get to be the classics in the first place. Plus while the Ossiarchs are compelling as a culture, their shared goals and ambitions, lack of internal strife, and just general newness make it harder to engage with them on the level of individual characters, where as the vampire counts with their backstabbing aristocratic politics and their sycophantic but scheming necromancer allies have individual personality out the eye sockets. I especially love Mannfred, as essentially the Warhammer version of Starscream. Also, the new models are fantastic. I have so many old skeleton warriors and zombies, and I was pretty happy with them. I didn't think there was anything GW could do to get me to replace them, but I was wrong. And the new blood knights? Spectacular. I do wish the book had done more to make the basic vampire lord hero stand out, something to encourage running more than one of them, and to make them feel more distinct from legion to legion. Something like the old system of bloodline powers. And speaking of bloodlines, I love the ones we have, especially the Vyrkos, but I'd also really like to see the return of a Necrarch style bloodline or the now defunct Legion of Sacrament - mechanically built around necromancers and death magic, thematically explicitly loyal to nagash, creepy, and feared/distrusted by other vampiric dynasties. Maybe we'll see something like that in a future book.