I had the luck of flying on a KLM 747 ICN-AMS; and a 747-400 (FRA-YVR) and 747-8i (JNB-FRA) of Lufthansa.
The KLM flight was in business on the upper deck, and it was a fluke because at the time only economy travel was company policy. However my VP took his sweet time to approve my travel, and by the time he did it only business was available. Somehow slipped through the cracks, and I got to fly in business. Poor experience all in all, with slanted seats, mediocre food, and a snoring neighbour. But the first time on the upper deck you can't forget!
Second time was on a 744, and I was booked in business. At the time Lufthansa was flying their 744s with an amazing upper deck configuration with 8 first class seats which also had a bed:
I went to the gate before my flight to get my passport sorted, and the friendly gate agent gave me a shiny first class boarding pass. I could not believe my luck, never got an upgrade before, and now this! Made my way to the first class lounge in Frankfurt, ordered eggs benedict and a glass of champagne, and waited to board. I slept about 8 hours straight on my own flying bed!
Finally I flew the 748 recently on the upper deck in business class, and it was a mostly uneventful flight, but the experience of going up the stairs is always amazing.
Fun fact, if you get to fly first class on the 748 of lufthansa, you actually sit more in front of the pilots, since the nose of the plane is way ahead of the upper deck!