I just bought one there earlier today, actually. I've got several domains with them. So I like to keep everything in the one place. I've also found them really reliable, with prompt and thorough customer service. The price was low, too. I bought a .com.au domain for two years for around 23 bucks. That's not bad considering some local ones charge quite a bit more than that. (That said, Netregistry, which has a good reputation, does sell them for about the same price.)
I don't want to sound unpatriotic, but I have heard some bad reports about a couple of local domain registrars. Technical issues, mostly. So I figure it's better to go with a massive and proven international company.