It really is worth $5 IMO. I bought a $60 game with new mechanics, art, music, ect. $20 just for about 15 hours worth of quests and some reskinned gear/few unique gear isn't worth it.
If you blazed through the main material I guess you could say that, but then again that was how you approached the DLC. Whether or not you enjoyed it and how you played it was entirely dependent on you.
I've been beta testing it since the 16th and I just got to the final quest in the main line. I've put over 70 hours into it, which comes with my playstyle, but if someone goes through the DLC (let alone the core game) at break neck speed, then it's no wonder that it didn't seem like much.
It's akin to someone playing on Novice and stating that the game didn't give them a high enough challenge.