I had an involuntary downgrade to Y on a trip to NRT, the flight was overbooked (I frankly don't understand how this is remotely possible in a wold where the Internet exists) but I must say my experience was not so bad.
Im relatively young and in good health, the Qantas staff handled the situation very well, with the staff at the lounge and onboard apologising many times over and bringing me things from the pointy end during the trip.
I was compensated to the tune of $400 and the difference in price between J and Y with the crew manager personally giving me the cheque during my flight.
I don't count this as a bad experience towards Qantas staff, but I do question the IT systems failures that led to it happening and hope those will be reviewed and fixed.