It's in. Multinoise is finally in the overworld. Ever since I saw what was happening with datapacks I was wondering if this would be the case.
As cool as the mountains and caves are right now, it still has a very long way to go. The performance hit and the jankiness and lack of polish is very apparent. Biome and terrain generation on the surface, putting it lightly, is very very rough. It is not hard to tell why this is an experimental snapshot.
The separation of the terrain and biome generation leaves me feeling... Mixed. With time this could be an amazing system that finally dethrones 1.7 from its 'same-5-biomes-for-thousands-of-blocks' tyranny. The notion of the separation is a wonderful idea, but I must say I am... Apprehensive. I am not seeing much smooth or flat land for builds, for example, and finding some specific terrain with a biome to match may be difficult. However given the level of polish over the past few years and Mojang's dedication to quality and satisfaction, I should hope this pans out in great ways.
As for the biomes and caves themselves... Wow. I mean it, just wow. They are wonderful. I hope we see some new structures to go with them and updates to old ones.
The monster spawning change is a very welcome one! No more do you have to spend time finding that one block that's light level 7. ...However, part of me wonders if we'll be seeing some special deep or abyssal monsters int he deepslate layer. We shall see.
All in all, excitement is in the air!