PCCW's wifi is one of the cheapest option of wifi connection all over Hong Kong, with 9,000 hotspots. The current cheapest plan costs only HK$48 per month, unlimited use for one registered device. No contract is needed, one month at a time.
However, when use, you will want to check if the coverage includes the places you usually will go:
When reading the coverage map, remember the signal is basically 2D!! That means, if you go up a building of 50/F, do not expect you can get any signal at all!!
For better coverage, many will use the 3G telephone network. The CONS is that it's more expensive and with slower bandwidth.