I’ve been voluntarily denied boarding (VDB) four times this year on United, netting me a total of $1,800 in United travel vouchers. Earlier in the year, I also turned down a $400 offer because the reroute involved a layover at ORD in the middle of a blizzard. I’ve been offered two bumps with Continental, one at $400 to fly the next day and one at $250 to miss Penn & Teller. Meanwhile, American’s never sought volunteers on any of my flights (although I’m 3 for 3 on same day standbys to earlier flights, possibly causing myself to miss out an unlikely chance of being bumped).
Apparently, my anecdotal data points can be confirmed with statistics; United’s VDB rate is bit over twice that of Continental or American.