I have successfully used them, but you have to be flexible. I booked acceptable* flights as soon as I could and then checked back frequently to change them when the flights I really wanted came up. In the end I got 2 F returns on the A380 for ~$1000, including a couple of change fees when better flights came available. It's possible. On the HKG route availability is posted a year in advance, and then a few days before if the cabin isn't filling.


*acceptable = not perfect dates, or from a nearby airport that requires a cheap positioning flight.