The "consume" ability from Xhana functions based off her magic, which means there's a bit of a feedback loop (the higher her magic is, the easier it is to land consume in battle, which then means it is even easier to continue increasing her magic). Once she can hit consume on enemies with full health, with some control + AP and mana restore support, it is not difficult to set her up to eat a lot of enemies in easy fights (e.g arena fights or using lure close to a portal relay for free mana restore in a low-level world).
It is slow but it is a reliable form of infinite growth, and the rest of her skills are also decent to make use of her very high magic (blink for positioning, ice path and flamestrike for multiple target strikes + some artifact like "world's end"), as far as "Uniques" go, this makes her pretty okay. Even if you are just playing casually without grinding it, using it once or twice every fight in a long playthrough adds up.
The "Demon bond" is obviously worthwhile if you plan on using both Xhana and Avy at all times, a toss-up if you plan on using only one of them, and awful if you don't want to use either of them. There is an armor artifact that disables all buffs and debuffs on the wearer, so you do have some flexibility and aren't completely fucked if you took the demon bond while not intending to use Avy and Xhana (just equip that armor on your MC if you got the demon bond but don't want to use Avy and Xhana to remove the debuff).