I had mixed feelings.
I found the armor to look kickass, and Tentacle Explosion is both visually satisfying and does descent damage, but as it is a "chance on hit" thing, it's only really practical if you intend to get up close and personal with enemies in melee combat - spell casters and archers would not be able to make practical use of this power.
Really, they could done any number of things - as with other items in this DLC, it could have given you the ability to use unique spells (perhaps a Tentacle-Explosion blast?) or, more obviously, decrease shout cooldown and/or increase shout effectiveness. Bonuses for mages makes sense, as the Dragon Priests were mages anyway, and a big part of Miraak seems to be making a deal with a being who seeks and collects knowledge.
Most of Miraak's stuff seems to be mostly eye candy - amazing and fun to look at, but lacking in actual power. Really, it seems the only thing that serves practical use is that sword that when you power-swing, it throws energy blasts.
I feel kinda let down. Bethesda had space to do something really amazing here, really creative, and they kinda squandered it.
Bethesda needs to make more proper mage gear - boosts magical power (preferably in ways that cannot be achieved through enchanting/smithing) maybe gives access to unique abilities, and, of course, robes. A mage in a big suit of armor hardly feels like a mage at all. (Maybe light armor, but it should still have a robe-like appearance, I say.)