Quite simply Demon's Souls is one of the best games of 2009. The game has sucked me in completely. I have been playing for the last week. Some people call it an RPG but I feel it is more of an Action RPG. Yes; you do level up stats and get better weapons but the story is pretty weak. The meat of the game is the combat. It is very tactical and when you die you always know why - you made a mistake. Death's never feel cheap (except for when you fall).
All the character classes can eventually do everything so you never feel like you picked the wrong class. The knights and soldiers do have an easier time of it in the beginning but the magic users get deadly quickly.
You have two states of being: Soul Form and Living Form. In Soul Form you are safe from other players but when you are alive prepare to be invaded. The meat of the online play is cooperative and versus play. In cooperative play you help other players slay demons. In versus play you try to kill the player whose game you invaded.
I experienced an invader tonight. It was exciting. I was just doing some soul farming (repeating an area over and over to collect souls; I do this on level 4-1 since you can get about 3000 souls in five minutes) when my game was invaded.
It was a long battle (at least 10 minutes) but I finally defeated him. It was a great feeling. The great feelings come frequently. Every time you beat a part of the game you feel great. Keep in mind though that this game is hard and you will die a lot but don't give up. You will also feel incredibly frustrated at times but mostly in yourself not because the game is cheating.
If you have been on the fence regarding a PS3 purchase I suggest you get one - the good games are here. Uncharted 2: Among Thieves comes out tomorrow as well. I bought my PS3 to play the first one and wasn't disappointed. I am very excited to play the sequel.