While flying to Taipei I had a stopover in Hong Kong. I figured I wanted to scoot into town and check it out. The word on the web regarding the feasibility of this, and how long it takes, was inconclusive. Well on the way to Taipei I tried out a practice run to the city without actually leaving the airport area: through security, change money, find transportation, stash baggage, etc. Is gave me the confidence, and some key info, to actually make the run on the way home. Here are my findings and tips, for the Hong Kong newbie on a tight time budget.
First off, with 4 hours in Hong Kong airport, a quick dash into the city is possible and, if you have never been there before, quite interesting and worthwhile.
Hong Kong airport, at least when I have been there, is very efficient and you can get in and out fairly efficiently. In particular, getting into the airport worked very smoothly. (I have seen substantial lineups getting out.)
The train/subway from the airport to the city is the way to go, and much preferred over a taxi. This is because the train is fast, immune to traffic jams, and cost effective, and the distance from the airport to the city is surprisingly long. The train is very modern and puts most Canadian, American and European public transit to shame. It even features an animated graphic that shows you just where you are along the trajectory ever moment.
If possible, try to have local currency on hand since finding the exchange and waiting in line to get currency can blow a good 20 minutes.
Good places to visit on a very time time budget are Mong Kok or Hong Kong (downtown). I think Hong Kong proper is a bit better since it avoids train transfer, and thus saves a few moments and the risk of making a mistake of some kind.