It's cute, but very forgetable, you know? She'd get lost and mixed in with all the other Hayley/Hailey/Haley's,
Callie's,
Kayley's, etc. Yes, it may not seem as popular on the charts, but it's concept is used all the time, making the name rather boring to me, I'm afraid.
My initial reaction, actually, was
Hallie from The Parent Trap with
Lindsay Lohan... I'm pretty sure
Hallie was the American twin.
It's a nice name, but I'd definitely prefer it as a nickname for
Harriet. I don't think
Hallie would be too immature either, especially if her real name was
Harriet.