(Warning: I have a hatred for Mia that is probably more intense than it deserves.)
I say MEE-ah, and I always want to preface it with a "Mamma" or a "cara." For me, it's too much like naming a kid Mine. In fact, it always makes me think of the idiotic seagulls in Finding Nemo who say nothing but "Mine? Mine? Mine?" as they fight over fish scraps. MEE-ah isn't a pleasant sound anyway, imo--too much of a mealy bleat.
I much prefer MY-ah, but that's Maia or Maya. There was one in my son's class last year, spelled Mya.