Overall a mid DLC.
If your primary interest is story, this DLC might not worth it.
If you like combo flexing or no damage run, this DLC worth it.
This DLC reward basically Megaman Buster (Leviathan) and Nero's Devil Trigger (Ultima Form) from DMC V adapted to Final Fantasy.
Leviathan is nice, skippable non-game breaking addition, while Ultima Form is, similar to Nero's DT, provide significant boost, can be unlocked after you got all Eikons but not necessary to clear the game.
No change to ending. It stays same. Really make you wish there's "Vergil" for Clive, so this game can have "Special Edition".