I have volunteered to be bumped by Copa on a number of occasions, successfully three times. In each, I was offered a $400 voucher, lodging overnight in Panama City, and the usual perks (meals, transportation). Twice I was invited to a free tour of the Miraflores Locks, which allows ships into the Panama Canal.
For me, an extra day added onto my vacation with some money for the my next vacation flight was a no-brainer.
I would think carefully, however, about how difficullt it will be to redeem your voucher. As of a year ago, it had to be redeemed in person with Copa at the airport (or at one of their ticketing offices). I live about 3.5 hours from LAX, so making a trip to save $400 was not worth it.
I did happen to have a flight through LAX which matched up with the Copa counter's hours the last time I redeemed one.
Also, while traveling out of the US, I tried to use my voucher at an office in Montevideo, but the taxes that were added for buying my ticket there were rediculous. I waited until I arrived back at LAX to redeem.
All in all, I would probably do it again. Who am I kidding? Of course I would! I live for this sort of thing.