If you wanted to leave the airport and assuming you're from a country that can get visa on arrival, then you do have time to go into town for a meal or a stroll. Normally if the airport is busy it will take you around 1/2 hour to clear immigration and another 1/2 hour, roughly, to get on the airport express train and the journey into the city. Spend 3.5 hours in the city then come back to the airport, again assuming you will already have your boarding pass and the luggage are checked through.

http://www.mtr.com.hk/eng/airport_express/intro_index.html

If you wanted to spend the time in the airport then IMO that's not too bad either. Lots of shops, both air-side and land-side, see below for the airport link.

http://www.hongkongairport.com/eng/shopping/index.html

I agree with the assessment and I think if you've never been to Hong Kong, it's worth a quick trip out to the city for a quick stroll.

Get the "same-day return" tickets on the Airport Express, ride to Hong Kong, then take Star Ferry across harbor to Tsimshatsui - which should be around 8pm for the light show across the harbor. Wander around the Avenue of the Stars, then grab some bites to eat in Tsimshatsui, a bit of (window) shopping, then head to one of the major hotels to ride the free Airport Express shuttle bus to the Kowloon station and take train back to airport.

Think that'd be more fun and enjoyable than staying at the airport.

I would agree that you should take the ferry across to Kowloon (Tsimshatsui) as it is very busy in the evening. Lots to see/do/eat/shop, etc.

thanks so much - we are coming from Sydney so I think visas will be easy to obtain - might look into heading out.
rgds
AC