I have not had any Nikon repair experience- knock on wood though.
However, I have had a friend have repair experience.
I think one of my friend's co-workers had dropped a 14-24 2.8 into water. The issue was that I think it was heavily repaired before that and it had gotten a new serial number. The thing is, Nikon did not track serial numbers, they just repair them for you so the warranty does not transfer that time.
I'm not sure if they track serial numbers now, but I know Sony does do that for the PS3. They knew that my PS3 had been repaired once before under warranty and knew about it the next time they fixed it. Not sure why Nikon didn't keep track of their serial numbers.
I don't remember if my friend got the zoom fixed, I'll e-mail him about how the story ended. But this happened about 2 years ago.
But the thing is, Nikon doesn't exactly have a stellar repair customer service track record.