Just beat Demon's Souls. What a wild ride that was. The King Alant fight at the end of the Boletarian Palace stage was the bitch to end all bitches. Unlike the boss of Dark Souls, it just takes place in a big, empty room, so there was nowhere that I could get behind to act as "cover". Had to get really good at dodging, and had to completely change my usual equipment load so that I'd be light enough to roll properly.
The ending to the game was much, much better than in Dark Souls though. I actually understood what was going on. Kind of an important factor. And the Old One was just surreal enough to creep me the hell out.
Having played them both, I think I like Dark Souls more, but both games had their obvious advantages.
Demon's Souls had, in my eyes, the better implementation of a story and a slightly better personality to it (which counts for a lot ). And that King Alant fight was a definite highlight for me (best one-on-one, human vs. human fight in the series so far, for my money)
Dark Souls had the better visual design and music, though, and made all the right tweaks to the areas of leveling up and equipment modifying.
Dark Souls 2 can't come out soon enough. I haven't felt this satisfied with pure RPG character development in a long time.