Most of it isn't worth it, although, the weapon and shield set can be used to get a quick boost.
Imo, the real dlc is the 3 extra dungeons, each one specializing in an aspect.
Flower Fantasia is your seed source as all the enemies here are the flying pigs that you kill with throwing items.
Omega Power dungeon is for plain raw currency, all flying sheep enemies. You may want to stockpile arrows before trying this place.
Whetstone Caverns for me is the big game changer. This dungeon has standard enemies with a conditional lock that you must kill all enemies to make the stairs appear. This place is full of upgrade tomes for your gear with standard Weapon/Shield/P Tomes for +1 and the + versions for a +3 instead. a typical run through here is likely to give you around +15 to a weapon, shield and undergarment.
So if anything it may be worth it for the DLC dungeons.
"If history is to change, let it change. If the world is to be destroyed, so be it. If my fate is to die, I must simply laugh." -Magus