I used to say Mee-buh, but a friend of mine called it Mih-buh so then I eventually just started saying it Mih-buh too. I think it makes more sense with Mibbians, but I'll say Meebz sometimes.
When I first joined, for some reason, when I said it out loud is was Mih-be-a. I put a second I in there, and didn't think about it until I was telling a friend about it. >.< I say Mih-bah.
I say Mee-bah. I think I like it better than Mih-bah. Whenever I'm not sure how to pronounce something I pronounce it as if it was a Spanish word. Maybe because it was my first language (though I speak better English than Spanish).
I've alway pronounced it as mee-buh, I don't know why, but I watch a lot of anime, and in do-re-mi 'mi' is pronounced 'me' so for me 'mee-buh' just fit.