After reading this, combined with the other positive buzz, I'm definately doing to have to grab the demo when I get home tonight. If for nothing else, to see if it will run decently on my machine.
One thing to keep in mind if you play the demo is that the final version has been tweaked. The demo ran terrible on my machine but the IGN review gives me some hope -
The first thing you'll notice is how nice it looks. The proprietary engine is the product of years and years of development, and it's got more bells and whistles than a Rose Day Parade. The list of features is as long as my arm, and they're all fairly clearly explained in the options menus. You can tell the game to auto-detect the best settings, then run a brief benchmark to determine how well it's going to play. Or you can tweak dozens of different toggles and sliders to your heart's content. The review rig's auto-detect settings, with 3.0GHz P4, 1GB of RAM, and a GeForce 6800GT OC at 1024x768, had 4xAF (anisotropic filtering, to make textures look nicer), but no AA. (I was able to sneak 2xAA in, but I don't know if the game would perform well across the board with that).
Soft shadows were also off, but almost everything else was cranked to the max, and the benchmark ran at an average of 39 frames per second. For what it's worth, I used my home rig to play the game (which went gold about three weeks ago) at my place of residence, and I tested multiplayer on my game rig at work, which has similar specs but twice as much RAM.
Anyhoo, FEAR looks like it has indeed been optimized since the single-player demo was released.
So don't be to discouraged if it runs like a hog.