I use the Moonlight Butterfly Horn. I am a pure Mage. I use it in certain areas where enemies don't hit too hard or have predictable tactics. It helps save on spells when you have a ways to go before your next Bonfire. Plus, it can be used with shield raised and has immense amount of reach. Only need 12 STR and 18 INT. Pretty basic for any mage. Shield sucks other than the current R2 ability scaling incorrectly. But, its up to you.
I used the shield on my last character and figured I'd give the horn a try on my INT build since I don't need the ranged attack of the shield since I have all of the spells, but it sounds like it sucks.
Glamador posted... I've tried to use it. The damage is mediocre but it's PURE magic damage, so it does more than the stats page indicates compared to mixed damage weapons.
But I find I do more damage using the Channeler's Trident on my mage. I'm sticking with that as my spear of choice, even though I prefer the more basic moveset of the Horn.
I have a character that uses the butterfly horn and one that uses the trident. Aside from the hilarious 2hr2 dance with the trident (best victory dance) I like the butterfly horn more. It only has one damage type and i'm pretty sure I saw somewhere that it is getting a boost whenever the patch comes.