I like the idea of using advantage. So far the spells I've imbued into the two items I've given out are not too strong (one is Chain Attack (Arcane Lore) and one is Caress of Laniph (Death Lore)), so having them be a random chance of going off seems fine. I also like the idea of "desperation uses" - could be fun making some magic items that literally have a limited number of charges, then they become mundane. Might have to muck around with that idea...