When the design is good enough?

It is really hard to say when you stop telling your designer "keep
working on it", be it user interface, or functionality. After a few
iterations you may start asking yourself -- am I just unreasonably
annoying? Is this good enough?

I found the answer from Apple's Steve Jobs:

"We made the buttons on the screen look so good you'll want to lick
them."

Fourtune, January 24, 2000