One thing that you will come to realize if you spend any time at all in Hong Kong is just how darn useful cell phones can be. If you are travelling with a group of people (or even a family that might go in different directions from time to time) having a functonal cell phone in Hong Kong can be one of those things that takes your vacation from good to great.
The good news is that companies such as 3 Hong Kong, 1010, China Mobile, and others have pay as you go cellular service offers for those of you who already have your own unlocked GSM band phones. Unlocked (or unblocked) phones mean that your phone is not locked to a single carrier. It is almost always possible to unlock a phone, contact your local carrier for more information. Basically, you just remove your home sim card, insert the Hong Kong sim card that you have purchased (the prices are very low) and you are off and running with your own Hong Kong phone and phone number. The last time I purchased this sort of thing at the Hong Kong airport 7-Eleven store it cost me $150 and lasted an entire trip. Remember, this is only for local calls, if you want to do long distance calls there are other options, including special long distance sim cards or calling cards.
For those of you who have smart phones or tablets with data needs, there are also short term data options available, which can give you access to the internet almost anywhere in Hong Kong. I say almost anywhere because phones work pretty much everywhere in Hong Kong, including in the subways, which takes a bit of getting use to. The only place phones never seem to work well is in the middle of your hotel room for some reason, not entirely sure how that happens!