First off, this is my favorite game of all time. I've had it for a long time in physical format, but due to it's drm it's unplayable on modern systems, so I bought the steam version. I was a bit afraid of how it would run based on some of the reviews that have reported problems, so I was ready to issue a steam refund just in case.
So here's the score: this game is flawless. I've seen some negative reviews and let me address some of them. First of all, all those who say it's unplayable with keyboard and mouse are idiots, no two ways about it. You control your speed via the mouse wheel. It's that simple. Besides, you can customize the controls to your heart's content. Second off, no, it doesn't have checkpoints. And you know why? Because it doesn't need them! It has a quicksave function! I swear, some people...
How about the game itself? It's the best stealth game of all time. Period. You have full feedback of how much noise you make and how much noise is in the environment, how visible you are, you get a lot of tools for dispatching or distracting your enemies, you have nightvision, thermal vision and even "electric" vision that let's you see objects that use electricity (useful when completing some of the objectives). It has a fairly intelligent AI. The game is a bit too easy if you just kill everyone in my opinion but it's damn fun. If you like the suspense of stealth, look no further.
The graphics still hold up to this day in my opinion. Some of the round objects are a bit angular, but it doesn't take me out of the experience, and the textures still look great. There's also a surprising ammount of color that, compared to a lot of modern games, are a breath of fresh air. The sound design is superb and the soundtrack just ups the suspense. The music is low key and dynamic, changing to the gameplay.
Some of the writing is a bit too convoluted for me. Don't get me wrong, it's good, but it's hard to follow, especially for the first time playing. You really have to invest your brain into the story or you'll get lost in it.
All in all, if you even remotely like stealth games don't you dare miss out on this one.