I've done it but had a very specific purpose in mind, I wanted to pick up some meds from Fanda that I can't get in Japan. I actually had less than 4 hours in transit but I know Hong Kong quite well. I also wouldn't recommend it to a first time visitor unless they really have a specific destination in mind that's close to Kowloon or Hong Kong stations and smartphone maps to navigate once on the ground. Another point to consider is inbound and outbound immigration can take a bit of time during busy hours (nothing crazy like entering the US or UK, but keep in mind), that doesn't apply to regular visitors as we use the e-Channel which takes a couple of minutes at worst during the most busy times. The good point is that a same day return Airport Express ticket is the same price as a one-way (100HK$, can be paid on credit card at the Airport Express counter).

Otherwise for the OP, HKG is a nice airport but nothing special to do like Changi unfortunately. Your layover is long enough that it might be worth while to get a day pass for the United Club. You can easily get better food for less outside the lounge, or even fast food if you fancy, but I like the lounge for a place to settle in and recharge before the next flight. Pretty sure the HKG UA Club also has showers which will likely be welcome after your long trip.