The Nini's are the ones I own - and LOVE. But, I would never call my feet narrow. I have normal-ish feet all over, with bunions on the toes therefore calling for nearly wide shoes at the toe box. I wear a half-size bigger in any ballet flat, including the Nini, to accomodate my foot widening as the day wears on - because there's little arch support. It is true the Nini's slip off my feet, but only for about the first 20 minutes or so, then my feet start sweating, since they are not made of leather, and inevitably swelling, and they stay on just fine.
GG - if your feet are narrower, you may want to try Cole Haan. The shoes are more expensive, but they are very well constructed and last and last and last. Plus they have the air nike technology in them. My second favorite pair of ballet flats are Cole Haan's. They aren't *quite* 8-hour shoes, because they are narrower for my foot. Many of their models cut right into my bunion and I can't wear them at all. But the pairs I have are constructed in such a way that part works - while they are just a tad tighter in the heel, yet not so bad they leave blisters - possibly perfect for you?