If you purchased the game then by all means use the DLC code and get the add-on, otherwise the $7 price tag may be a bit much for the small offering. That being said, fans of the game most likely won't be disappointed.
Overall, this DLC is around two hours, which is perfectly reasonable for it's price tag, but that all depends on just how much Darksiders you want. With the main game dropping at least 40-50 hours if you do it all, this extra dungeon doesn't bring nearly enough unique aspects to make it seem worthy of the cost considering how much you already got. As a bonus for pre-ordering the game though, it fits it purpose nicely, adding more content to an already outstanding package.
Argul's Tomb adds one new quest, three new dungeons, and takes less than three hours to complete. The quality is what you'd expect, but ultimately it's not worth the cost; that is, unless you pre-ordered Darksiders II and get Argul's Tomb for free.
So, is Darksiders II: Argul's Tomb worth playing? It's only about an hour long, and the exclusive weapon contained within is a set of armblades that drain Wrath with each hit. While they're pretty effective when partnered with the scythes that drain life from opponents, running out of Wrath was a problem that never really came up in the course of the game, so-perhaps ironically-nothing in the Argul's Tomb proved more valuable than the club I acquired by dealing with Argul himself. For players who can't get enough of Death beheading monsters, Argul's Tomb will be a welcome addition, but anyone who completed the main game and set it aside without further thought won't find anything worth slotting the disk back in.
Argul's Tomb (to be included in the Wii U version) packs two donjons, new enemies (even though they are but new skins on already fought 3D models). It's a little light for its price, especially considering there's no new puzzle-types, but its hour and half additional gameplay is still very much interesting to play for the fans. Too bad it's a separate campaign and not something weaved into the main one, though.
It's certainly disappointing that there's almost no story (I think there are three conversations in the entire DLC), it doesn't really seem to tie into the main quest in any significant manner, and the gameplay has been pared down to Darksiders' button-mashy combat, but Vigil has shown a willingness to learn from their mistakes; hopefully, we'll see that again in future DLC.