I have.
When I was in Ecuador for a year, I lived at 3,200 metres above sea level (nearly 10,000 feet), so after nearly a year of being there, I felt ready to actually undertake something like that and climb the Cotopaxi, the second highest mountain in Ecuador, just shy of 6,000 metres in height. I can tell you that without that one year, I would have never had a chance in hell of making it, and even then, it was the single most physically demanding thing that I have ever done, but it was all worth it to stand atop the peak and watch the sun rise above a sea of clouds.
Sadly, my friend who was with me was the one taking photos and a day later, when he was taking the bus back to the city where he worked, he was robbed and the pictures we took were gone forever.