Finally tried this last night. The problem for me is not that the game is hard, but that it's punishingly unforgiving. I think the comparison with FFXIII is great. Both games let you get in way over your head, and have monsters that can one-shot you.
In FFXIII, you get one-shot, die, reload immediately before the fight, try it again defensively. If you get one-shot and die again, you learn your lesson, reload immediately before the fight, make a mental note that you can't defeat this monster yet, and move on. In total, you've lost a few minutes.
In Demon's Souls, you get one-shot, die, lose all your experience, start over at the beginning of the world, and everything respawns. You re-clear everything, try the fight again, get one-shot again, die, start over. Except Demon's Souls has so much that can potentially one-shot you, that you don't learn your lesson. So you re-clear and die again, and again, and again. Then you look up the FAQ to find out that you're not supposed to go right at the fork. So then you go left, get one-shot, die, restart. Go left again, kill the monster, progress further, get to another fork. Go right, almost die, run back to the fork. Go left, die, restart. Die again, and again. Look up FAQ, find out you're not supposed to go left. So now you're back to where you were 3 hours ago. Oh, and the FAQ suggests that you should just re-roll, because you picked the worst class. $%#!
I love that the game is hard, and that so much can kill you so easily. It instills a great sense of fear and frailty. I hate how much the game punishes you for making a mistake, and how much time you waste if you don't follow a guide. Re-clearing gets boring. Oh, and not having a pause is inexcusable for a single-player game. It's like they designed DS to suck quarters at an arcade.
Edit: And yet, despite all that, I think I'm going to go and buy it. WTH is wrong with me? I feel that DS preys on your stubbornness. It pounds you into the dust, and kicks you when you're down, but as you're spitting out teeth you can't help but think "I won't let this damn game beat me!"