Fantastic space horror.
The story, which I think draws a lot of criticism on the negative reviews, is not the most crucial part of a sci-fi horror. What is important is the setting and atmosphere, which Callisto Protocol absolutely nails.
That's not to say the story is bad, it's not, it's very much on theme with what a sci-fi horror should be. Survival is the mission, as it would be if you were in Jacob Lee's shoes, and while trying to do so, you discover what led to the events in which you're now trying to survive - it doesn't need a complex plot, and I enjoyed it as it comes.
I found the combat mechanics fine and enjoyed the close-quarters combat as it added to the tension knowing that you can't just kill enemies from a safe distance. However, later in the game enemies do become spongy unless you can enlist the aid of certain scenery to dispatch them quicker, which I often sort out to do.
Callisto Protocol is the first recent release in years that I've been fully invested in. I played and completed the main story over a weekend, as I just didn't want to put it down. I will now buy the DLC as i really enjoyed the base game.
I played via Game Pass and on Hardcore difficulty.