Is the Artorias of the Abyss worth it, you ask?
Yes, it is!
But you should make your own opinion about it.
So let me give you some basic info.
Playtime is around 2-10 hours, largely depending on players skills and how much of the new content they want to explore.
Story is about three of Gwyn's Four Knights, with Artorias, playing the lead, the fall of the ancient kingdom of Oolacille and the nature of the Abyss.
Gameplay is difficult and unforgiving, as it should.
The DLC was intended for seasoned players, that have finished the game. While many may disagree to this, it is a stonehard fact, that enemies in the DLC are tough and bosses prove to be a challenge, even to high level characters, as they are quite different in contrast to other bosses.
Additionally, the DLC gives you acces to new armor, weapons and spells, which are very good. One spell in particular is all the 'rage' these days.