Yes I have, and I would again, but as is so often the case, it all depends on the details. To maximize your gains, one needs to have a clear strategy, a game-day plan, an iron will, and a readiness to jump when the ball is in play.
You may giving it up way too cheaply. $600?! Pffft. Six hunderd bucks is nothing to sneeze at for a minor inconvenience, but the numbers can get much higher. Call me when you're talking 4 figures...
Back when I was "commuting" for work on flights regularly, I took careful steps to make this work. I had a good idea of the flights that were most often oversold (because I booked those flights, I knew that many were business travelers trying to go out and back on the same day). My plan was to book flights that were frequently oversold, but also that had later flights available on the same day that would get me where I really needed to go, on time. Also very predictable: flights get full and oversold just before and after long weekends and around holidays. One can easily see how busy a flight is by checking available seats (the seating chart doesn't always give a full picture, because it only shows you assigned seats, and not everyone picks their seats in advance, but it's a good starting point; the airline websites will tell you current ticket prices, and often adds the friendly note "only X seats left at this price." There are other tools available, too).
Did you know, you do not have to wait until they announce they need volunteers before you get on their list? Yes, most airlines/gates actually maintain a list of volunteers for bumps. This is a double-edged sword: you can approach the gate agent as soon as they arrive and tell them you would be willing to take a bump, I've never had an agent refuse to put me on their list (or start a list with my name at the top). The downside is that if the bidding goes up, you may have sold yourself too cheaply. So it pays to have a good idea of how full the flight is, how full the next flight is, overall demand, etc.
Everything's negotiable. Doesn't mean you will get what you ask for but, you can try. I've asked for (and received) all of the following:
- A guaranteed seat on the next flight out (or a later flight I specified)
- A seat in First/Business class
- A specific seat (eg window or aisle, not a non-reclining seat back by the toilets)
- A voucher for food in the airport (helps if your delay will be a long one)
- A voucher for food or drinks on my flight
- A voucher for a hotel near the airport (when your delay will be overnight)
- A voucher for a free, round-trip ticket anywhere that the airline flies to (that was a nice one)
- Frequent Flyer points
I wouldn't expect to get all of those things at the same time, but when they REALLY want your seat, you can get a lot!
That said, if I really do need to be on my original flight (to make an irreplaceable connection or other time-critical event) no price is high enough to get me to give up my seat. But if I've got some flexibility -- especially if I'm on my way home -- then I'm willing to play the game.