I used the service of 世界通 for less than 2 months. It takes too long time to get connected and the carrier charged some unconnected calls.
Currently I'm using the service of 亚洲电讯(AIC). The connection is quite fast, but the voice quality is so so. Many times it broke during the conversation and I had to re-dial. (At least I've met 15 times! Averagely it broke once every two calls.)
The worst thing is although AIC is my long distance telephone carrier, I cannot dial long distance numbers directly!!! I've made at least 7 phone calls to AIC, they just didn't care my complain. So the only way left for me is to switch off.
That's my experience. Hope it's helpful for you and I'm eagar to hear yours.