FPS Perfection. At it's finest.
Either way, developers should make more games like this more often.
Why?
Because this game is the perfect FPS. In each mission, you are presented multiple paths to complete it. You can go stealthy and try not to kill anyone, you can go stealthy but kill some people, you can go all-out shooting, etc. Use your imagination! Anything's possible here. Maybe except absurdity.
But wait. This has been done multiple times, even to this day. Hold on a second! Your actions actually have CONSEQUENCES! Yeah! If you do something, people will comment on it later in cutscenes. And heck, maybe some AI will react differently depending on your actions in the past.
I've only played it onto 4 levels late, but I can already tell people to buy this game. This is abseloutely what I was looking for; and I don't even like stealthy games that much. This is a golden exception.
As you progress, your character gets more developed, and there's a wide variety of skills to choose to upgrade from, making the gameplay even more varied.
By the way, the storyline is also great. You are a goverrnment pseudo-super agent of the UNATCO (the good guys) stopping a terrorist threat named NSF (the bad guys). I won't say more so the story isn't spoiled. You can't miss it.
There's also great humour in this game. Sometimes you might stumble into your allies talking about humorous stuff in those times where you're not out there in the battlefield. It's entertaining to watch. Speaking of talking, the voice acting is also great. The voice of the "emotionless badass" agent is very well done and so is the voice of... well... everyone else. They could get more voice actors for the generic soldiers, though. Seeing two people with the same voice can get awkward.
Anyway, what am I saying?! Go buy this game now. It works perfectly under Windows 7 64-Bits with the latest patches. It's very cheap considering it's age, but in all circumstances, you will NOT regret it.
In short: if you like RPG/FPS hybrids with an immersive story, lots of free choice and overall awesomeness... go get this now.