If you're looking for a challenging FPS this will certainly satisfy you
Of course, the graphics can't hold up to modern times, but they still look good and work in creating a great atmosphere.
The atmosphere is one of the strongest points in this game, it feels like a mix of Medal of Honor meets Doom and is always tense, with fierce and challenging foes and the possibility of them jumping around the next corner, and they can eat quite a load of bullets before they go down, which sometimes can be really annoying.
On the standard difficulty the game already is pretty challenging and sometimes even unfair with enemies all of a sudden appearing and attacking from behind. Though this kind of ambush is rare, already a frontal attack of the enemy can really make you break out in sweat, especially when you've just managed to get past one intense difficult fight and to the next without having reached a checkpoint yet. Checkpoints themselves are also pretty rare and their implementation is a bit strange, sometimes you have two side by side without having really fights in between them, and then again you have to force your way through several hard battles without one in sight for ages, which means, if you die in between you have to re fight a whole lot of already seen battles and enviroments, sometimes over and over.
The story itself, while neither a masterpiece nor very interesting at least serves it's purpose to keep you motivated to play, may it be just because you want to see what comes next, gameplay-, enviroment-, or atmosphere-wise, but it's also interesting to see what kind of enemy types are thrown at you next, and they're quite varied and often attack in interesting, and also challenging combinations which force you to constantly think your strategy like throw a grenade to the front rows and keep them occupied while sniping away the far away enemies and get back to cover as quick as possible so your health can regenerate.
Also the game features an interesting range of weapons, from the classical assault rifle, over a sniper rifle to the alien weaponry. One thing that indicates how old the game is though is the aiming for someone who's used to more modern day shooters feels pretty broken, you have to switch back between aiming and standard fire and sometimes it seems like the aiming crosshair get's stuck without being able to aim normally again for a few seconds, which can cost your life more than on one or two occasions.
So the conclusion is, though the game has some balance and gameplay / fairness issues, if you're someone who always thought this days shooters are too easy and forgiving, and you're looking for a challenge or always wanted to shoot down fierce and hard alien foes in a great atmosphere, you can't go wrong with this game. But as a casual gamer, this game might be not for you.