Some friends and I bought it back when multiplayer was still on the table. Then that feature got sadly shelved, so the same happened to Subnautica. Now it's just sitting there in my steam library looking at me judgingly. But it's going to stay there for now.
You're missing out. It's a wonderful game. I understand that some people (raised by the games of this new age) can only enjoy a game with friends and playing alone is a waste of time. Multiplayer games are like sports. Great fun and an opportunity to socialize. But good single player game is like a good book. It gives you something wonderful and deeply personal. You learn something about yourself.
Anyway, what I wanted to say is that I just spent two nights in this thing and it's bloody perfect.