You can get good photos from inside the terminal, though it will be through glass.
There is no dedicated spectator's area in HKG. Unfortunately, there isn't really any good place to take brilliant photos in Hong Kong, unless you take a (non-scheduled) boat to a remote bay on the island next to the airport.
Pls let me know if you have any further queries.
See my homepage for a comprehensive guide to spotting and photography at HKG