Dark Souls, or DkS is a great game to play if you are looking for something that will punish you for making even the slightest mistake.
This is unfortunately not the game to play if you are seeking fair and balanced PvP due to all of the Save File modifiers that are running rampant with infinite health or some other thing though (Something the developers have said to be fixed in Dark Souls 2, coming March 11th 2014..which I pre ordered a long long time ago).
THIS IS the game to buy if you are looking for a rewarding experience that will make you feel like you accomplished something when you finally figure out how to beat the boss that you have been banging your head against a wall over the past hour for!...or some bosses longer :)
If you want a game for its in depth story..well, this is and it isn't that game. Yes, it has a story...but it's loose and depending on how you play, it will be given to you broken and if you avoid certain spots, you won't get the fully story. Heck, the developers said they didn't make the game with any story in mind..they just kind of created a basis and built from there and branched to this and that. It works mind you, just don't expect to have the full explanation handed to you.
Buy this game and experience it and while you're at it, get Dark Souls 2 pre ordered (I love me some pre orders ha) since you'll be wanting to play that game after you complete this.