If you are a fan of Demon Souls, then this incredible sequal is a must.
Dark Souls is very similar to demon souls in many ways. You can customize your character anyway you wish. If you want to be a spell caster or a warrior, wear different types of armor, use different types of weapons, it can be done. You can leave messages to help out other players. You can even summon players into your world to get help on a boss, or invade other players to take their humanity and souls.
Those people rating this game with insanely low scores, I wonder if they even played the game to be honest. Some people complaining its unfair? Not at all. If you die in this game, its your fault. Others complaining about lag? I have not lagged in the entire game a single time and i've played the game for over 70 hours now. This game is absolutely incredible and if you were a fan of Demon Souls, you will certainly be a fan of Dark Souls as well.
In short, if you are a fan of Demons Souls or are a hardcore gamer, I strongly recommend buying this game. It will keep you busy long after you beat it your first time through.