It's quite tense, atmospheric, and it feels good to play skillfully. Unfortunately, it has just enough problems that I wouldn't recommend it to someone.
First of all, the entire gameplay through each of the levels is quite fun. It reminds of more of the type of shooting you would do in counterstrike as opposed to quake. It's all about peeking corners and out-reacting the enemies. Pretty much everything you can shoot gets destroyed in one hit, which makes the combat more satisfying and fast-paced.
Sometimes, an enemy will notice you before you notice it and utter a cry, giving you a warning and a bit of time to respond to it before it gets the drop on you. Those sort of situations are definitely fun because you actually know what you could've done differently if you take damage. It keeps you on your toes, too, because you're always trying to listen carefully for enemy cries. I think it adds to the tense atmosphere in a good way.
Other times, however, you won't be able to get the enemies without completely leaving the corner and you'll have to strafe around the room trying to dodge the turrets and other enemies. The turrets' turning and shooting behavior is very unpredictable which makes these sort of forced situations very frustrating because there doesn't seem to really be a reliable strategy for dodging them. Furthermore, I found it very difficult to shoot while strafing in this game - it seemed like the game was always stuttering a lot when I was trying to do this, which is frustrating.
Also problematic, for a game that's all about aiming and shooting and lightning fast reflexes, it absolutely should not be having occasional lag/stuttering. Unfortunately, it has that and it seems to have it right when enemies are attacking me or coming into vision. It's not very freuqent, but it's enough to be annoying and mess up my concentration or ability to track what is going on enough to start taking damage. I am not alone in having this problem, so don't assume you're not going to have this issue either.
Finally, the biggest issue that makes me not want to recommend the game is the difficulty spikes. Specifically, I'm talking about the level 4 and final bosses. The level 4 boss is ridiculously challenging and doesn't at all fit with the methodical, patient playstyle of the rest of the game. Going in blind, you're almost certainly going to just die in a few seconds. Further compounding this issue is that, once you die, you don't get to quickly try again to see how you could've improved (which would be a very satisfying game loop IMO). You have to then carefully plod your way back through the first four levels. I found myself getting impatient because I just wanted to have another go at the boss, and I would end up dying before reaching it because the game really requires a patient, methodical playstyle. This is, of course, highly frustrating, and it feels like the game is just trying to waste my time. It's like, I've already demonstrated I can handle the first stages, just let me go back to where I was at the edge of my abilities, where games are most engaging, dammit!
The final boss is even worse in this regard. It's absolutely overwhelming when you first encounter it - there's so many enemies, new elements, sounds, etc... that it's just impossible to make sense of it your first time. And you have to play through eight stages again to even get to try to understand the boss after you inevitably die. While you're playing through the stages, you're not really getting better against the boss either because you play through the stages with such a different playstyle than you would need to use against the boss.
Overall, the game is intense, atmospheric, twitchy, and responsive (compared to the bullet-sponge type FPS gameplay in similar titles). If you like the sort of FPS gameplay in counter strike and are looking to spend a lot of time on mastering something like that, you may like this game. But if you're someone like me who just wants to have a fun gameplay experience, you probably will find this game more frustrating than fun because so much of your play time will be spent doing things that you already feel you've mastered.