Just completed this, thoughts below:
+Decent graphics, aside from some shoddy indoor textures
+Solid 120fps with the exception of the odd stutter when entering a new area (SSD might fix this)
+I really like the way you can press a key to see glowing footsteps to show you where to go next, so much less intrusive than having an objective indicator glued to your HUD the whole time
+Combat is generally satisfying with a variety of ways of killing people
+Time manipulation feels less contrived than I was expecting
+Storyline is generally pretty good - at least it is reasonably original although it can be a bit tricky to follow in places
+I like the RPG elements i.e. upgrading weapons and abilities, progresses steadily throughout the game as mentioned above
+Some good, simple puzzles
+Majority of the game transitions well i.e. you move steadily through the game with good continuity and not many places where the is a random change of scenery.
-As I said, the textures leave a lot to be desired in some places
-Felt like it could have done with an extra boss fight, you do two in the first half of the game and that's it I think?
-Plot is occasionally confusing
-Multi-function controls i.e. pickup and reload are on the same key so sometimes you accidently reload when wanting to collect an item

This is a real bugbear of mine in PC games where you have over 100 buttons at your disposal.
-At 8hrs it is a little on the short side, although it didn't feel that way.
Overall I was pleasantly surprised, it reminds me of Bioshock (although not quite as good and with a bit less polish) and I would say it is one of the best 'lesser known' FPS I've played in recent years. A helluva lot better than say, Timeshift for example.
Can't remember how much I paid for it last year but it must have been cheap as according to Steam transactions I got it the same time as Fable The Lost Chapters and paid £5.48 in total

Definitely keep an eye out for this in the sales if you are into FPS but want something a bit different from the usual modern day or sci-fi fare that gets rolled out.