This game has been great since even before its release on Steam. It features some really interesting mechanics that make the gameplay very interesting and always keeps it from being stale. I understand this game won't be for everyone, but if you're into tactical shooters or stealth games, I feel this game will be right up your alley.
I've had the game for 7 years now and am still yet to find any game which can stand up to this. A lot more tactical than Rainbow 6 that's for sure.