I'm getting to be an older gamer. My reflexes aren't what they used to be. And to put it mildly, my adventures in Demon's Souls, much like many others (but unbeknownst to me at the time), started right out of the gate with many ass-woopins. This was discouraging and feeling that this game was a great departure from King's Field, I stupidly shelved it until this past January (2014).
5 years since release, this primarily single-player game still has an active player base as evidenced by all the talk about it on various gaming sites that I've seen over these last 5 years. So I fired up the game again with an open mind to the notion I was going to die a lot and gave it a real shot. And I'm glad I did.
There are plenty of user reviews here already so I'll just comment on the perspective of a King's Field fan for any others who were fans of that series. Demon's Souls does have that same 'King's Field' vibe to it. But instead of the slow and stiff combat of King's Field, Demon's Souls is fast and fluid and much more challenging.
For me, what made King's Field so fun was the open world exploration aspect of the game. But Demon's Souls too offers a great deal of satisfying world exploration along with plenty of worthwhile secrets to find and challenges to overcome.
However, unlike King's Field games, Demon's Souls world is structured like a 'hub.' Soon into the game, you will find yourself in a place called the Nexus. From here you can access 5 different 'worlds' in any order you like. Your restrictions on where you can go and when are largely dependent on whether or not you can survive the inevitable onslaught of bad guys whose 'soul' purpose (sorry, bad pun) is to stop you. While I prefer more of a 'Skyrim' Approach to open worlds (just a big expanse of land that goes off in every direction from the get-go), the hub-like nature of Demon's Souls still allows for a great deal of freedom.
If you like open worlds, lots of customization to your character, and tough battles that require tactics and strategy to overcome, you'll love Demon's Souls.