Have King HS on my MTB and have had them on my road bikes as well. Bombproof in its truest sense. Like Ronco- set-it and forget it. Butter smooth after 10 years, no play. Have had them on road bikes, and like other posters, just keep swapping them to the next frame over and over.
Have King SS hubs, same quality, more maintinance, but wonderfully made.
Can I tell the difference between a new, properly adjusted King HS and say an FSA or a Ritchey? No. But give it a year or two and ask the same question. The likely answer will be the King will last longer. I have had Tange HS's pit in under a few years. Same for Ritchey. I know how to turn a wrench, its not mis-adjustment that reuined my gear.
In this society where upgrading is taken to be more fun than actually riding the bike you bought.... buying one headset and never replacing it may seem counter-intuitive.