I wouldn't typically spell it this way, but one of by BFFs just had a little girl and named her Millee. Her maiden name was Miller and her dad's middle name was Lee and he passed away 10 years ago. With the meaning, it makes sense to me. Otherwise I say Millie all the way.
Re: Milly/ Millie spelling
I had a great-aunt with this name. I'm fairly certain she spelled it Millie.
I think this is a great name, though slightly less enchanting than Tilly.
2012