Bought it yesterday and started on Hard and ended on Chapter 19 yesterday after about 9 hours. So far, the game's been amazing. It's one of those things where the graphics are SO good, you literally are popped right out of the game when you realize 'Holy shit! This is amazing! Someone actually took the time to do all this just for this view!'.
I cannot believe the attention to detail everywhere in some places where you only briefly visit.
That being said, Hard doesn't seem to be as difficult as I remember it being in Uncharted 1 but maybe it's because I'm used to it. There seems to be an excessive number of propane cannisters, grenades, and the guns all appear to carry more ammo.