This is a HUGE airport. Give yourself plenty of time to spare before departure. You may need to board a tram to get to some gates. You'll probably need at least 20 minutes to walk from the check-in area to the gate (excluding tram time). Security is TIGHT and SLOW. We got stopped for carrying an aerosol can (shoe cleaner/deodorizer) and the guards refused to throw it away. They insisted on giving us a receipt for it even though we told them we were not going back to HK in the next 2 weeks and that we were late for boarding. It took the guard about 20 minutes to write a simple receipt as she had to copy some of the english words from a cheat sheet!
Hong Kong can be a smoldering oven with very high temperatures and close to 100% humidity, you need to drink lots and lots of water to keep you from dehydration.