I love the name Sadie. We may name this baby Sadie, except that everyone has a freaking opinion about this name and my H isn't sold on it. And it's really annoying.
If you think about it, anything can be used as a dog's name. Katie, Macy, Ginny, etc. My dog's names are Willy and Macy but I don't go up to people named Will/Willy/William or Macy and tell them they have dog names. That's so rude.
I've only ever known one person named Sadie and she's a really cute girl that's my age with a great personality. It's a great name and if you love it, go for it!