If I'm being honest, I used to not like this name. It sounded weird and unpleasant to me. But I recently met a girl named Franny (short for Frances), and I started to like it. It's cute. I do not like it as a real name but love it as a nickname, like short for Frances.
Haha, I'm a man named Francis and my boyfriend uses the nickname 'Franny' a lot and personally I think it's adorable. I might be a little weirded out if an acquaintance called me Franny, though.