My children have mastered this word. Although now Kaylie adds to it. She'll say "No, Don't want it" or if she doesn't want to get off the swing at the playground after swinging for 30 minutes "NO, Swing!".
Cavan says it to Kaylie everytime she tackles him and takes his toys (Yes she's a bully, we're working on it). He also says it to the dogs when they bark.
All those times I don't mind. Its the times when I ask them to do something and they say no that we have a little problem. So we're now working on the word "YES" and using that as often as possible.
Now let's just hope they don't pick up any other words that may be uttered on occasion that we would prefer them not repeating ;)