My DD was thrilled when Delta offered a bump with upgrade on a flight an hour later and $200 travel certificate. When she got to the next gate, she was told that flight was overbooked and she was 7th in line. Delta had offered many voluntary bumps to this sold out flight. She had an 8 hour layover before the next leg and thought she would be fine, but it was scary.
Be careful before taking a bump--make sure the next flight has seats!