Hello,
I went through HKG last month with some hours between flights.
On the way I had 4-5 hours coming in from SYD in the evening and paid for the Plaza Lounge (about $40) I wanted a hot shower and some quiet time, I had to wait 1.5 hours for a shower, it was not that quiet really busy actually and then also booked a massage, about $40 extra which was excellent. Food is below average and there are only local beers and soft drinks available. This is nowhere near a Qantas/BA business lounge! So it depends what you are really wanting?
On the way back I had 7 hours during the day so opted to go to Hong Kong Island, via the train, fast clean and you can leave your case at HKG so no luggage. Loads to do, great foods and short tours.
Second thoughts if you are flying Qantas why not see if you can buy a Lounge Pass on ebay, something I did not know but picked up from this forum (they normally sell for $40-50), QF have a great lounge over there and as long as you are flying QF it should be no problem - however I do stand to be corrected by more senior guys who 'know the ropes' better than I do.
Hope this helps