I've had a few. A couple on Y to J on United JFK - LAX. Also a few on upgraded J to F on that route.
I also had upgraded with an SWU on United from Y to J on NRT - JFK and had an aisle upstairs on a 747. When I got to NRT they had moved me to a middle seat in J downstairs. I politely told the woman in the Red Carpet Club that this was a bad thing to do to someone who flew as much as I did, I was a 1K at the time. She said they would try to get me a better seat. I did some duty free shopping and checking back into the RCC another person at the desk told me they had moved me from the middle to an aisle downstairs and were still looking for an upstairs aisle. When it came time to board the original woman was the gate agent. She recognized me and said, she would check upstairs again, nothing unfortunately she said. Then she said wait, typed a few things and handed me a first class boarding pass and thanked me for flying United.
I had another on United following an equipment change. I was flying LGA-DEN and they downgraded the aircraft and I lost my seat. I went to the special services line to rebook and the grown man in front of me who was on my same flight was having a total tantrum, yelling at the agent. I came up next and politely asked what he could do for me. He said he would put me in first to ORD and wouldn't pull the upgrade cert and first on a three class flight ORD-DEN. He also gave me a $300 voucher. Being nice pays.
I have also had some on BA for being an AA executive platinum. The two best were premium economy to J on LHR-BOM and premium economy to J on PVG-LHR.