I played Demon's for 40 hours, but I could never get into it like I did with Dark Souls.
Sure, the mood is more "oppressive", but Dark Souls is just hands down a much better experience. I hope the next game will have the open word/bonfire mechanic, because it makes exploration so much fun.
It's kinda hard going back the consumables/MP system after playing Dark. The game is also faster with less sense of weight than DkS, which may or may not be a plus to some.
There are some areas where Demon's just shines, though. Latria cough cough.