This game is pretty much the videogame mediums Messiah. I've heard nothing but absolute praise for it. I'll probably end up getting it for the 360 since the game seems very atmospheric and that goes well with a nice sound system.
As far as playing the demo itself? Personally, I already know the game should be good, the genre itself isn't anything new (so I know what I'm getting into) so I would be avoiding the demo (not that it matter since my 360 has a RRoD). This is mainly so I get that "so fresh and so clean" gaming experience when I pop in that lil' DVD into the 360.