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I played the demo. It has three stages, they are like 5-10 minutes long each. The stages are simply selected areas from the first part of the game. There are a few enemies you can fight. You have some weapons and ammo, I don't remember exactly.
Basically, you can see what the first area of the game looks like, and what it feels like to run around and fight basic enemies.
In the demo you won't get to solve any puzzles (there are many in the main game), or collect plugs and open security boxes, or mix items. Obviously, there is a whole story going on that you can only get if you play the main game. You also can't make any choices (there are several paths in the main game based on the choices that you make).
Despite that I'd say that the demo is a pretty good indicator of what the game looks and feels like. If you like the demo you'll like the game, if not then you won't.