I've probably been sucked into the hype of it, making me see it as better than it is but Christ, it's good. There are lots of areas to see but it all takes you along the same path in the end.
I got to a bit where there were lots of goodies in the middle of a room. I thought "What's the catch? There must be a catch" I started picking things up then when I got the shotgun the lights went out and left a spotlight on. The surround sound is used really well because I could actually hear them coming from behind out of the darkness.
I think I died twice over the hour or so I played it. I decided to have a break when I was using telekinesis to throw grenades back at the bloke. I only had a bit of health and Eve left with no first aid kits or Eve rechargers. I was meaning to go back to it but I got sidetracked as usual and ended up doing other things.
The environment is so detailed with loads to explore. There was a dead cat which I found in the demo. Not sure what that's about. It may not be truly unique but it's a fresh environment. How many other FPSes are there set in an underwater city?
I don't know about 4th best game of all time but it must be the best of this generation.
Edit: You posted while I was replying.
@James, when you start getting into the game, you'll start to realize that it's a living, breathing world. It's not a simple "corridor" shooter. And the decisions you make, however subtle, determine how the game plays out. You'll see. Enjoy the experience! I've got to get back to playing it...
I think I roughly said that in this post.
It might not be entirely coherent because it's got me excited.