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I bought this game a few years ago and one day decided to play it again on my modern PC, Windows 7, 12GB RAM, core i7 @ 3.6GHz and GTX460 and found it way too tough as the enemies seemed to be 100% accurate and really made it difficult to play as you were getting low on health due to the extreme accuracy of the enemies. You could play it but you had to be so fast to shoot the enemy first it made it more frustrating than enjoyable.

Is the GOG version exactly the same or has it been tweaked to make it more forgiving on modern hardware and more playable?
I remember i left the game unfinished years ago just for what are you telling. I dont know, i didnt really liked this game, it´s hard i will give it another chance. I played for long time just to arrive "Mars" as i´m a total fan of the topic, but i even not arrived there... the game gets better on Mars?
I only have the GOG version, so can't say for certain. It seems like a fairly easy shooter to me though. I'm playing with a mod that removes the hud, so I don't even have a crosshair on screen.
Remember to use that adrenaline (bullet-time) folks, and even wait for it to regenerate in between firefights. It's a tough game, but not that tough.
I tried on easy and was too easy. To the point it was boring. Then tried on medium and... too hard. But here is the trick to enjoy the game:

Play on normal and think you are playing some kind of Max Payne. Use the adrenaline mode as much as you can. There are some levels at the beginning which are very punishing, because the enemies are in the worst possible location. This changes later, and you are also aware of that, so you will be better at spotting them.

If you don't like the level design, don't worry, as the levels improve later. However they keep being linear, but it's not so noticeable.