How often? A number of times over the past 10 years, various loyality levels, both KLM and AF, most recently today - an on-board "upgrade".
A young mother was causing "chaos" while boarding and talking to the purser (she was saying that herself - "I'm sorry for causing chaos"), then I hear 5E..., 5E?...man, that's next to me...
So here she goes her and her infant... but then the stewardess moves in saying "Would you mind "moving forward" sir so she can have more space?... and the purser will assist you with a different seat?...." Of course I would NOT mind, you're welcome!"
Ehh, I do miss Euro-Business, I used to upgrade quite often when I had no status and the price was right, I'm one of those guys who will take 2 empty economy seats next to me over 1 fancy american one but occupied, so that worked for me just fine. But now - with status - it's difficult to justify it. Although recently I've adopted a system - for each complimentary upgrade I will purchase another one.