By the fates, my Cleric needs this.  She's Strength-based for melee, and as she continues to level up, she keeps failing Dex-based saving throws from spellcasters.  What good is the healer when the healer goes down??

Does the advantage rolls count for spells with AOE (Fireball, Cone of Cold, etc....)??

@Ronll, Yes, all saving throws you make as part of a spell you get advantage on, no matter if it just targets you, or if it's AoE

Yes

My party just gave this to the already invincible Asaka who is immune to non magical attacks. WTF!

Doesn't cloak of protection give the same advantage but to all saves (not just magic) and plus +1 to ac as an uncommon magical item?  This is subpar unless you are looking for trade fodder at the rare level.

cloak of protection gives +1 to AC and all saving throws, while this gives advantage on saving throws against spells.

I see!  That does make it much more powerful than I first realized.

Does this work on creature abilities that require saves? For example the eye rays of a Beholder

I would say no because Beholder eye rays and other similar monster abilities are not technically spells. They are abilities innate to the creature. 

This is a difficult subject to cross, but personally I view monsters Spell-like Abilities as being essentially magically based. Therefore anything not physical o would rule as being subject to the mantle's protection

This is the same problem some people have with Wizards dispelling clerical "magic" when in fact it is their God's power, or dispelling an Artificer's "spells".

So my personal ruling is anything that can be remotely described as "a magical effect" falls under this umbrella. Beholders especially

“This shroud, a creation of the daelkyr Orlassk, is interwoven with hundreds, maybe thousands of minuscule crystals that force even magical energy to a grinding halt.

Toward what purpose the stored energy is funneled, only Orlassk—and perhaps the mantle itself—knows…”

Quote from ZaRx584 >>

cloak of protection gives +1 to AC and all saving throws, while this gives advantage on saving throws against spells.

 IIRC from a mathematical dice rolling standpoint advantage typically equates to a +5 bonus over time. I can see why this item is rare compared to the cloak of protection.

Why is your cleric in melee

way to many monster stat blocks use spell like effects in place of actual spells, rendering this magic item mostly useless. 

Can this stack if you have advantage because of your race?

no, in 5e advantage and disadvantage can't stack. you can't roll more than two d20s for any check, attack roll, or saving throw, except with the elven accuracy feat.

Advantage and Disadvantage cannot stack