Its free already, don't buy this. It was always free from GOG for the original edition of PST, which you get anyway when you buy the enhanced edition. If for some reason they removed that, then I'd give this a negative 5 stars for taking content away in order to squeeze you for more cash. GOG should remove this from the store.
Sunless Sea is one of the most addicting, engaging, and supremely dark experiences one can have in a game. Sure, its a bit like a choose your own adventure with RPG elements, but it does it so perfectly. The game design is very very old-school. You will have absolutely no clue what you're doing for the first hour or so of gameplay. Luckily, all the gameplay concepts are very simple to grasp. Read the short tutorial to learn the very few keys required, and just go explore. Don't be (too) afraid to die, because death has benefits, and it will happen from time to time if you're not extremely careful and lucky. Like everyone else says, the graphics and atmosphere are stunningly gorgeous and disturbing. The world is right out of Lovecraft's nightmares. Even more disturbing are the events that unfold throughout the story. This is one of the darkest games I've ever played, right up there with Planescape: Torment, and although the games are different styles, I enjoyed it just as much. My only knocks against Sunless Sea is that I wish the music played more consistently because its quite lovely and eerie, and that the in-game journal is as obtuse as is possibly imaginable, its almost completely useless. Because of that, I highly suggest you take hand-written notes. I did say this game is very old-school in its implementation. Don't expect your hand to be held on the zea, ever. Its a very unforgiving place, but yet you'll yearn to discover more and more. That's the whole point. Discovery while pushing through a constant and slowly crushing feeling of dread. If that sounds fun to you, by all means buy this and it's Zubmariner DLC, you won't regret it. If you wear (metaphorical) diapers while gaming, then please bugger off and go play Call of Doody.
