I just spent 50 minutes with the demo
I followed Marconelly's instructions closely and had no trouble booting this up with HDLoader.
I didn't skip any cutscenes or codec sequences, and I was taking my sweet time exploring the environments and checking the menus. I only got up to the point where you encounter the first two enemy soldiers, I was laying on the ground with my camo meter reading 90% and the soldier was walking right by without detecting me, but then I got the idea to shoot him with the tranquilizer gun, and as soon as I touched the analog stick to turn around in order to face him, he was alerted so I froze and didn't touch anything hoping he'd dismiss it, but after a few seconds he freaked out, starting firing at me and called for backup

I ran off and quit the demo because I need to sleep, but not before posting some impressions here
The game is gorgeous, during the intro I didn't read any of the subtitles because I couldn't take my eyes off the upper portion of the screen, the graphics are jaw dropping, better than MGS2 IMO.
I was sad that there's no option in the demo to invert first-person camera, I use invert in all games and it's difficult for me to adjust to this.
I collected a few food items, a snake, a tree frog, and some fruit.
WTF is up with the alligators not dying? I fired about 10 bullets at one of them with the .45 gun, it became a bloody mess but didn't die! Also when tranquilizing them, they wake up and attack you the second you walk over them.
If you spend any amount of time standing in the mud swamp, you drown and die, must keep moving.
The life-detecting sonar is so cool, you'd have no idea that there's anything living around you from the normal camera view point, then you activate the sonar and there are dots all around, enter FPS mode and sure enough you can see them crawling in the grass, ready for you to kill them for food if you choose to.
30fps doesn't really bother me, in fact I totally forgot about it and didn't notice it until about 15 minutes in or something, then it came to my mind, and I moved Snake around while trying to remember if it looks/feels any different than MGS2 (I'm not a framerate expert like some of you), and I noticed that yes there is an obvious difference, but apparently I'm blessed with it not bothering me if I'm not thinking about it. Sure I'm still hoping the final game can pull off 60fps since there IS a difference, but if it doesn't, I'm not going to lose any sleep over it.
There's A LOT of screen tearing in FPS mode, especially if there's a field of tall grass in front of you. Now that bothers me more so than the 30fps. I hope it's not as bad in the final game.
The codec conversation between Snake and The Boss was quite emotional and touching, I can see that this game will probably have a much stronger emotional impact than the previous two... Well MGS did have it's really strong moments, but MGS2's "drama" was mostly cookie-cutter to me.
It's so strange seeing Big Boss (Snake) as a total rookie in the beginning of this game, he's much less experienced than the Solid Snake we know from MGS and MGS2... Such a contrast from the badass Big Boss we always heard about from those games.
There's still so much for me to see and do in the demo, but that will have to wait till tomorrow, I'll update with more of my thoughts about it later.