Not quite as good as the first one in most aspects, not terrible though. Exploration is still really fun and the biomes are just as creative and cool as the first game. In a kind of bizarre move, Below Zero focuses on story a lot more than the first game and also changes the method of storytelling drastically. Subnautica told its story through audio logs, database entries, and the environment. It's not a particularly amazing story, but it leaves a lot to the imagination, and it bolsters the themes of loneliness. You also don't really need to engage with it if you aren't interested due to the unobtrusive nature of the storytelling. This game tells it's story through mostly dialogue and the inner thoughts of the player character, which isn't inherently bad or anything, but not that fitting for a Subnautica game, especially because the story just isn't that interesting. I do still love a lot of the new creatures in this one, but I will never forgive them for replacing the reaper leviathan with what is essentially a giant shrimp.

Improved on the original game by just about every metric other than the writing. Story isn't super impactful, but it's more involved than in the first game, whether you think that's good or bad.

I think a lot of people's judgment on this one is clouded by a lack of novelty due to it being a sequel -- of course the leviathans aren't as scary when you already played a whole game learning how to not be afraid of them. The only reason the world doesn't have the same sense of awe and discovery is because you already spent so much time in it.

Definitely worthwhile playing, maybe even prior to the original, but also if you just want more Subnautica.