My family and I enjoyed Below Zero a lot as a complement to the original game. If you liked Subnautica, you will probably like this similarly. Below Zero uses essentially the same mechanics with the addition of some new ideas we liked, and the flora and fauna design is just as wonderful as the original. However, this game is less atmospheric and contemplative than the original (with a lot more dialogue per exploratory trip), and we found it to have some issues with pacing and cohesiveness. Also, to be clear, Below Zero is substantially smaller.
I recommend this game to anyone who enjoyed the original Subnautica, but I don’t recommend it as an introduction to the Subnautica series.