I own it, and love it to death. It will really kick your butt if you don't be careful, or learn the ropes of the game quickly. I would recommend it only if you can handle the frustration :P It is possible to beat once you get the ropes and learn the game, but even then it is difficult.
The thing is, though, that your goal in Demon's Souls shouldn't be about beating it. It's not very story heavy and mostly only rewards you in gameplay rather then in twists and turns of a story. Think of a game like Monster Hunter. You have a "base" where you buy stuff, weapons, items, level-ups, etc and then select a level to go to and return when either you die, touch the teleporter, or beat the level.
So, it's not impossible, its just really hard. I would recommend it if you are ready for a challenging, dark action-rpg.