I enjoyed this game a lot! I think I had just about as much fun as I had with the original Subnautica, though there were also parts of it that I felt weren't as strong.
Below Zero is a lot more story focused. I personally liked the the story and the characters, but I do feel that a lot of the open-ended survival aspects that made the original so interesting had to be pushed aside in favor of them. The stakes don't feel as high since you're no longer playing as the lone survivor of a crashed spaceship trying to get off an alien planet.
There's plenty of nice QoL changes (pinning blueprints to the HUD, no longer being able to accidentally eat an item you're trying to put into storage). There's also some cool new base-building items to play with.
The environments are beautiful, though the map has been significantly sized down. Since you have to keep hopping around the map to progress the story, the feeling of "vast scary open ocean" unfortunately fades away somewhat quickly.
The creature designs are great! They have lots of charm. My favorites are the glow whales and snow stalkers. The leviathans are also really scary.
Despite the couple flaws, I really liked Below Zero overall!