Tried it last night Oz to UK, with a very good dial up connection (50k). Person at the other end didn't even notice any difference until told ! Nice clear line, slight delay. Adjust you style of speaking, ie pretend you are using radio and wait until the other party stops talking before you start and this minimises it.
At AU$0.03/min well worth it as once you have your internet connection, there are no hidden charges like with the prepaid calling cards, ever read the conditions on these ?