I always fill mine in, even if I'm wearing minimal makeup and just running to the store. It's amazing how put together it can make you look.
I also have black/brown hair so it's a MUST for me to fill and darken my eyebrows.
I have light brown hair and my brows are a bit lighter than my overall hair color, so I think darkening and defining them really make my face look great. I seriously will go outside with just brow pencil but no other makeup!