I was with Optus. After the bullshit databreach stuff, just wasn't worth sticking with them. Not so much that they got Databreached. It was that they didn't give a shit that people's information was leaked. All they did was offer 12 months identity protection which they probably didn't pay a cent for as it was a marketing deal.
That along with the constant connection issues I had with them. Vodafone has even worse service, but at least they're honest about it. When I have a connection issue with Vodafone, they say that there's a problem. When I had an issue with Optus, I would call customer support and they would tell me that there is no issue. So I would call mates that work for Optus and they would tell me that there's an outage caused by an Optus fuck up.
I have two mates that work for Optus. Even with those issues, they always tell me how Optus is the greatest ISP ever. I tell them that if they spend a week working for any other ISP, they'll change their stance.