I'm the type of gamer who doesn't mind that a game require a player to move at a slow, methodical pace to be successful. In fact, I much prefer it to mindless run-and-gun action. I suppose  that is why I've been enjoying Red Dead Redemption's multiplayer so much, because the crouching/stealth mechanic rewards those who want to be patient and take their time. I suppose I enjoy that style of play because my twitch gaming skills are pretty horrible, which makes pretty much all online FPS's not a fun experience for me at all.
 
That said, would I enjoy Demon's Souls?

Hmm, I guess when I look at how people were playing the new RdR coop missions for the first couple of days, as miners, rushing in to get killed immediatly, paying no attention to the lone marksman behind them (me) taking out snipers and dynamite wielders for them, and then you look at how fast they learned, theres a parallel there in terms of the game teaching you some patience.
 
Or, how easy it is now for me to knife a cougar or even a grizzly. By learning their patterns. Paying attention to the wildlife around me announcing a nearby predator etc.
 
But make no mistake, the overall challenge level in DS is a lot higher. And if your dead. Your dead :)

Do you ever want to throw your controller at the TV? 
If yes, Demon's Souls may help you buy that new HD TV you want. 
If no, Demon's Souls is a great game that requires methodical pacing and quick reflexes in tight corners. 
As for a comparison to RDR..no idea. Gave up once i got to Mexico.

Yes, Demon's Souls is right up your alley I'd say.  Be prepared to do a lot of "retracing your footsteps" so to speak.  The game does not change very much once you do a level.  That is to say that if you go through an entire level and remember where every enemy was, you will be prepared for that level if you ever go back through the level again because all of the enemies will be in EXACTLY the same places.  The enemy placement is rarely changed.  Variables only exist if you play online.
 
I really enjoyed Demon's Souls - I haven't played it in months but when it first came out, I couldn't stop playing it and when I wasn't playing it, I was thinking about what I wanted to do next in the game.  Its brutal nature was addictive for me and I loved taking down the many black phantoms that entered my game and cleaning up their souls :)
 
Be prepared for frustration - this game has no mercy!

Eventually, you will know enemy attack animations so well that you can rush through a stage, timing each hit and block perfectly.  It's a fairly fast-paced game that requires the right tactics going into each situation.

Slow and careful in terms of advancing through the level. Quick and strategic during battle. Be mindful of edges and the lock-on system. I can't tell you how many times an enemy has rolled or dodged off a cliff right as I was swinging, taking me along with him!