These days. Maybe 20% of the time on AA and QF... A bit more on CX, they have actually apologized at least 4-5 times when I've been stuck sitting in the seat I paid for because the cabin is biz cabin is full and they are unable to Upgrade.

In the old days when I used to exclusively fly economy... Once on Emirates, Twice on UA, twice on Aer Lingus, almost 50% of the time on old Continental before it merged and again regularly on BMI. This was when you could charm your way up to the next cabin and knowing people in the airline mattered.

On BA in 5 years only once, and it was an mid flight re-seating in CE courtesy of a flirty male cabin crew.