I can't decide if the AI is very good or inept. They hid well, aimed in the right spots, threw grenades when frustrated, and were fairly challenging. They even chased me down at one point when I had run away to miraculously recover from my bullet wounds, a la Gears of War.
At another point, though, I crouched underneath a small pillar, stood up, and plugged 2 of the 3 guys. The third moved round a bit to avoid being hit, but stayed in the same general area.
Do I like the game? You bet. My only visual complaint was the water effects when walking in the river. Very PS2 looking, although "wet Drake" was kinda surprising.
It's fun gameplay, and if it isn't a day one buy, it will be on my Christmas list. I may see what I have to trade in, as I'm not prone to spending money on myself this time of year.
I'm a little sour on the "miraculously heal from bullet wounds" sort of thing, though it's all fiction, I suppose. What's the alternative? Random health packs strewn about a pacific island? It's a catch-22 in that design spot, I'll grant them that. Nobody seemed to hate Gears of War either, I suppose. Or Halo.
I can't help but think it might have a somewhat fun online multiplayer mode based on the gameplay. I have no idea if they're doing that, or if this is a solely solo game. Sure seems to be the latter.
My only complaint about the demo is the lack of narrative. Flying, plane crash, shooting. It's important, and nice, to expose gamers to the gameplay before buying. It's also important, though, to grasp people in the narrative and make them want to buy the damned thing to find out what happens. Being told to press L2 to glimpse at some Blair Witch on Voodoo looking stuff is NOT narrative, IMO.
Overall, it's a very good game, and I'm looking forward to playing more of it. It's not perfect, of course. But, hey, perhaps I'm just not the kind of naive gamer to say "ZOMG! GTA/Mario/Metroid/Zelda/Halo/GoW is a perfect 10 that can't be improved upon at all!"