You really, really shouldn't need a walkthrough to beat the game. It ruins the challenge, and the challenge is a large part of appeal, in an era when games have quite simply pandered to the lowest common denominator (even the "Very Hard" modes). (Disclaimer: it's fine to use a walkthrough if there's one item you just can't find, it is after all still an RPG; but using it for strategies means you aren't developing your own, which is lame.)
I myself didn't find Dark Souls as difficult as Demon's Souls, not by a long shot, but that could be because I played the same class again, so I had all of those strategies pre-developed, and they're a big part of the challenge. Plus some of the discipline is the same in both games - by discipline, I mean never rounding corner without your shield up, never heading out into an intersection without looking both ways first, etc. - and after getting instakilled the first hundred times that way in Demon's Souls, you won't be in Dark Souls. Plus some of the boss/speciality fights play out similarly. ...So I'd say if you're playing Dark Souls for the first time it's just as hard as Demon's Souls, but if you've played Demon's Souls it won't be (and probably if you've played Dark Souls first now and you go back to play the original, it won't be as difficult).
So yes I play it specifically for the challenge. Very few games really make me think these days in order to come up with some coherent strategy, and this one does. Plus the atmosphere in both games is top-notch. They remind me of the original Diablo that way. The story is easily miss-able if you aren't paying attention, or you don't bother to connect the dots, but I love stories like that. Especially if they have a lot of ambiguity. (Same reason I loved SMT: Nocturne.)
They're not for everyone - my roommate has no interest in them because he doesn't want to die a million times over and over for tiny incremental gains, it's not his idea of fun - and there's no shame in that. But for people who hate having things handed to them (success - all too often the game is too easy; workable strategy - it will have some navi-esque character yammering in your ear telling you what works and what doesn't; the plot - some NPC stands there and explains it all) it's heaven.