I'm liking it so far. I'm a few chapters into, already completed several contracts. The level of choice you have as a player is phenomenal. For practically every situation you're either given a choice (do this or do that), you may discover a choice (eavesdropping, conversing, or finding notes give you hints at alternative routes or non-lethal options), or you're left to your devices on how to handle it (the "what happens if I flick that switch?" approach, as I like to call it
).
Overall the gameplay feels like Bioshock meets Thief. It's not a shooter, it's more of a stealth/platformer first-person game where you have the option of shooting. It's tough to describe because it breaks so many conventions but also sets new standards for its ambiguous genre.
The PC port is extremely well done. The interface switches on-the-fly to match to your preferred control scheme (radial menu for gamepad and hotbar for KB+mouse). I have faced no major issues (other than the one I've mentioned earlier about offline mode not saving settings) and it runs very smoothly.
I'm about 6 hours in (what with exploring and progressing the story) and I'm looking forward to uncovering more of the story.