Adding some of my experiences to this dated thread.

We arrived at the airport before midnight just to research the overnight options at HKG. There were a few people lounging on the chairs near the check-in area. The seats are roped off and cleaned in stages throughout the night. There is a coffee stand open 24 hours near the CX check-in with 3 free internet stations for customers. I visited the “Nursing Rooms” and they were not at all contusive to resting. The multi-faith “Prayer Room” provided no comfortable seats.
My assumption is most of those lounging outside of security could not check-in because of the airline they were flying. CX had check-in throughout most of the night and also has check-in the day before at the Central Station for those using the AE so I would recommend going through security no matter what time it is.
Once through security the expansive airport has several options. First the smaller downstairs portion of CX’s Wing lounge, for Biz, FC and elite PAX, is open past midnight but keep in mind they really do not intend it for PAX departing later in the morning. In fact the attendant acted as if the lounge was closed before he finally let us in but warned we would have to leave before the last flight left.
At 1:45 AM we were asked to leave as they boarded the final flight. The attendant was not all that friendly considering one of us was flying FC but at least he was accommodating and I can understand them not wanting people in there who will fly out in the morning.
We used one of the free internet stations located throughout terminal to kill time. Throughout the airport, sheltered from the walkway, are several areas that nave nearly flat lounge seats with a padded vinyl cushion for your head. This is probably the most comfortable thing I have seen provided for free at an airport. The closer to security the more crowded the lounge chairs are. Most areas are located on the airport directory. Some are not listed. For example, the lounge chairs near gate 60 was very empty but I am sure that changes later in the morning as that is a busy gate.
The services of the Plaza Premium Lounge and the Plaza Shower and Relaxation Lounge provides services 24 hours. From as little as $10 for a shower or up to $60 for a long nap, shower and refreshments. I did not go in here but someone in this thread mentioned the facilities as not that great but for the cost it certainly beats the Regal Hotel which typically charges over $250 per night. When I was there on a Wednesday the facility was completely full but I think they do take reservations.
At 4:30 AM we returned to the Wing. I was given the impression that they really only wanted arriving PAX that are transferring to other morning flights. People departing who are coming from Hong Kong were invited at 5:45 AM. However, they did let us in. At 5:45 AM they opened the top portion of the Wing with most employees not reaching their stations until 6:00 AM. Again the reception staff was not all that friendly. If I was in charge I would require them to be much more welcoming, like, “Thank you so much for flying CX FC and enjoy.”
The cabanas opened promptly and an attendant was on hand to take me to an available room before 6 AM. The shower and amenities are pretty nice but the room is fairly cold so probably not very contusive to actually sleeping. Too bad they no longer provide refreshments in the cabanas.
Table settings at the Haven were very nice however the breakfast offerings were not all that great. Dim Sum items were decent and the staff was very attentive. Noodles are not provided until 10:45 AM which really disappointed the Asian I was traveling with who prefers noodles for breakfast. Throughout Asia many noodle houses were open very early for breakfast: pho, hu teiu, won ton noodle soup, khanom cheen, etc. so I am a bit disappointed by not having noodles available, but as I would learn on the return trip, the Dan Dan noodles seemed too spicy to me and I can handle spicy. Both times there were more food items provided in the non-FC section so check it out before you leave.
The staff in the FC section were quick to offer a drink. IMO, if I had a choice I would prefer the Wing during the afternoon or evening but it was very nice to have it available before my early morning flight.
We took a very long walk over to the FC section of the Pier lounge which was much more enclosed with similar food offerings. All the relaxation rooms at the Pier were locked already so they must be that good but I did not get to preview any of them.
In general I was impressed with a few things about the lounges: I have never seen an airport lounge as nice as these, I was very impressed with the fact that they were every bit as accommodating to me even though I was just a guest of a FC PAX (I was in Biz class), and the wait staff were very anxious to assist.
Well I typed this up very fast so I am sure there is more to talk about so I will revise when I get more time.