Love the game, but it's indeed a system killer with details cranked up. Most games can't really beat-up my PC, but this one had NO problems bringing it to it's knees hehe. Aside from some spots with massive slowdown (again, settings intentionally all cranked up), the rest of the game played pretty nice and smooth though.
I was initially a bit confused about the low/laggy performance.. thinking "Hmm.. this doesn't look as good as Halflife2, but yet it's way laggy/slow framerate!"... but by about halfway through, the graphics really started to shine. It's apparent the second half of the game got the most attention to visual detail. The levels, lighting, surface detail and atmosphere are just gorgeous and incredible looking.
My only beef? Too short... or perhaps it's just too good! I played it pretty much non-stop and beat it the same weekend I got it on the default, medium difficulty. I compare this to about 2 weeks with Halflife2... so it was either how compelling it was to keep going.. or just a tad short.
So.. I give it a 9/10 overall. I haven't played the multiplayer yet, but that's next. The single-player firefights are absolutely some of the most intense and enjoyable I've ever played.. and the enemy AI is most definately some of the best I've experienced.