This was quite good. It's very rare, in my experience, to find an game that balances adult content with an engaging, compelling plot. Even disregarding the sex scenes, Ouroboros is very well put together, with an intriguing premise, deep and believable characters, and a surprisingly complex story for how simple a world it seems to be. The gameplay, while somewhat simple, is satisfying and presents a reasonable challenge; and the adult content, when it is present, fits naturally with both the story and the characters' personalities, and frequently serves as character development in its own right.
There are relatively few faults to be found with this. I personally would have liked a way to skip the sex scenes (after seeing them once, there's nothing new there); and there were a few small quirks that stood out excessively, such as the fact that Atreyan asks the same question each encounter with Viliana after using the shrine. Nothing that would detract from a five-star rating, however, or make me less likely to recommend playing it. Which I do.
I did purchase the expanded version. The additional content is worthwhile, if only for Atreyan's hilarious antics after enough resets; the additional dungeons are enjoyable (and make use of mechanics that are not well-explored in the main game, such as elemental weapons); and while the sex scenes did nothing for me, that's only because porn as a whole doesn't; they're well-written and original. My only complaint about it is that the ending is unchanged; after the required additional dungeon, it doesn't quite fit as well as it did in this version.
Really the only thing that I feel is missing from this game is an exploration of the outside world. While I realize that's out of scope for the game as it stands, it is what the entire plot was working toward. A sequel that goes into why all this happened and what's waiting for the characters afterward would be quite welcome.
Overall one of the best games I've seen on this site, and worth checking out even if you're not a fan of the genre.