Worth the price on utility alone
The Warden's Keep DLC add-on for Dragon Age is an easy buy if you plan to put a large amount of time into the game. At 560 microsoft bucks, you get about an hour long adventure, several extremely useful pieces of loot, and afterwards access to a vendor, blacksmith and most importantly an item storage chest. I would have paid double for this infinite capacity chest all by itself, as it became invaluable for storing rare and powerful equipment I could not yet use, or wanted to preserve for whatever reason. Thus Soldier's Peak became my frequent stopping point during my 30+ hour playthrough, and between it and the basic camp services your party management needs are completely covered.
The Keep is accessible immediately once you get to the overworld map and I would recommend it as the first stop before truly beginning your epic journey. The difficulty scales to your current level so I had no problem doing this mini adventure right off the bat. The other reason I recommend hitting it first is because of several very powerful items you receive. The Warden plate armor specifically is the best armor I ever came across in the game with very low stat requirements to boot, and my main warrior tank was wearing it all the way to the finish. There's a couple of great weapons too, as well as access to two unique skills for your main character. I believe these are tailored to your class, as my mage received an ability to trade health for mana, and an attack that used health to deal damage. The health to mana ability was extremely useful at many points in the game (despite having a simply goofy looking animation) and really helped preserve my supply of precious lyrium potions.
There is a healthy amount of content in this package that well justifies the price of entry. The adventure was fun, the rewards were great, and the Keep maintains usefulness throughout the entirety of your playthrough. What more could you ask for?