This has been discussed here many times before...
For PC to PC it is free.
For PC to phone it is significantly cheaper than "standard" IDD services (by which I mean dialling 001 or '+' from your mobile), but more expensive than most discount methods of calling IDD. For example, PCCW 0060 has for many years now offered me HK$0.17/min to UK, USA, Canada, Australia which is more or less identical to Skype's Euro0.017 without the hassle of having to be in front of my PC. Other services (for example, calling cards from various companies) can be even cheaper, and some companies such as HGC offer a flat rate "all you can use" tariff to certain countries of HK$68/month
http://www.hgcbroadband.com/idd0088/index_e.html
(but bear in mind that HK$68 would buy you nearly 7 hours of calls to the cheap destination using other methods, so you need to be a very heavy user for that to make sense).
Personally, for the UK, US, Canada, Australia the PCCW 0060 offer is as close enough to free for the amount I use that I can't be bothered to shop around to save, say, HK$5 per month.