The girl and I returned from Bali last Sunday, via Jakarta. Denpasar Airport is like something out of the 70's and has limited lounge facilities. However, the 'Prada' lounge is the Garuda lounge (I don't think there are others!) is adequate, however small and noisy. If planning a meeting, you can ring ahead and reserve a section.
If travelling Executive Class, you will be handed a lounge invitation upon check in.
There are many lounges in Jakarta, every asian airline has a different one. I counted no less then half a dozen.
Hope this helps.
PS. As for Garuda's new A330-200 J Class, the lie flat seats are a huge step up from their old hardware. I'd highly recommend SYD - DPS via CGK just to have the newer aircraft.