In my experience, compensation varies between airlines and really depends on how badly they need volunteers. Usually, it starts out at $200. I don't ever fly U S Air, so I don't know how they do it. Sounds different than the airlines I normally fly.
My DH, DD and I were traveling home from Italy (DD's orchestra trip) and waiting at the gate in Detroit for the final leg of our flight back to MSP. Apparently, there were two large groups of high school students booked on that flight (one of them my daughter's) and the flight was way overbooked. The compensation started out at $200 and was raised in several increments until it reached $800. I think the gate agents were getting pretty desperate. They must really hate to do involuntary bumping. Most of the seats were taken by the kids and of course, they were not allowed (by the chaperones) to volunteer. I wanted to volunteer, but my DH didn't. As it turned out, I'm glad we didn't. My cold kicked in big time right after we landed and I was miserable for a few days and very glad to be home.
Anyway, I don't know if they ever got enough volunteers. I would not have wanted to be one of the last to check in that day! My understanding is that those without seat assignments or those who check in late are the ones that get bumped if there aren't enough volunteers.
I prefer the $$ vouchers over the free ticket because the $$ can be applied to any flight/fare for a year. No capacity restrictions. Wish there were more ways to use them electronically, though. Have to go to the airport or an airline's ticket office to use them. That's not always very convenient and you can't always get the good internet fares if you show up in person.