Dark Souls was such a massive achievement that I think it actually led to the unfortunate consquence of rendering its predecessor, Demon's Souls, obsolete in the eyes of many gamers who haven't played it. It's not uncommon for me to hear that someone has played and adored Dark Souls but does not see the point in buying a game that they assume is simply less refined and less interesting.
That's a shame. For as opaque and weird as Dark Souls feels, to me, Demon's Souls is an even cloudier and weirder and eviler experience. That hazy nightmare-feeling that Dark Souls gets is somehow stronger and more frightening in the first game. It's a slighter experience, to be sure - if one can call a fifty-hour game "slight" - but so is Alien a "slighter" experience than its sequel and I think you'd agree that it's worthwhile to see both.
Next Tuesday, you can download Demon's Souls for twenty dollars over PSN. If you haven't played it yet, please do.