Dark Souls demands your dedication. You can't just pick it up, spend a couple of hours learning the mechanics, and expect to successfully explore a set of addictive but contrived, player-centric features, as in most other RPGs.
There is a very lengthy learning curve to this game, and its steepest slopes come at the beginning; judging from some of the other reviews, I would guess most of the naysayers stumbled on those slopes. I can relate; many times I moaned in frustration at having to repeat large chunks of a level, and many times I couldn't see how to beat a particular boss. But if you stick with it I promise you, you will be staggered at how much easier it can get.
The game achieves this because its mechanics are so tight: movement, combat and the interface are all responsive and precise. This is important because, though you'll shout at the screen when you die in the same place for the umpteenth time, deep down you know it happened because you screwed up and not because the game is cheap. That's why you'll keep coming back; you know you can do it, and when you do, you feel vindicated and empowered.
That's the game's real triumph: when it all starts clicking, the game isn't getting easier because your character has leveled up, but because YOU have become a better player. The depth of satisfaction that comes from brushing aside some of the game's tougher fights, when on your first playthrough they took you ten attempts and a summon to beat, is an unequaled feeling.
I've focused on the difficulty and why it's a good thing, but it should be said that the game is brilliant in most other respects: the world is gorgeous, with a menacing beauty plucked from a fever dream. Item descriptions and the sparse dialogue of sinister NPCs hint at depths of imaginative lore. There are at least four distinct ways to build your character, each of which makes each playthrough a very different experience. The boss fights are without question the finest I've ever tackled in gaming, both in the monster design and the gameplay (make sure you get the Prepare to Die edition for the fight with Knight Artorias). The game's only shortcomings are occasional frame rate drops (mostly in Blighttown) and the too-common lag issues that ruin an otherwise excellent PvP system.
TL;DR: you may be put off by the difficulty at first, but stick with it at least until you've beaten Anor Londo. If you rise to the challenge, Dark Souls will push you to achieve feats you thought impossible in those first few hours. You'll look back on yourself taking your first hesitant steps out of the Undead Asylum and actually feel a sensation of personal growth. No serious RPG fan should miss Dark Souls.