Optus internet at Home (Melbourne suburbs) .... but Telstra for mobiles. Both have been reasonably painless.

But, for whatever reason, [Telstra] started putting a $5 refund on my bill each month.

Finally got a call back on my complaint about doubling my mobile plan cost. The Filipino guy said it's because my old plan has been 'discontinued' -- Telstra code for 'we are not making enough money out of you'.

So I told the guy that my NBN cutover is imminent and I'm going to have to decide who my future telco is and I'm not happy with Telstra doubling my plan cost for zero benefit. Then he said" "OK we will refund you $10 a month".

It is a blatant penalty for loyalty, just like the insurance companies that whack up your policy fees every year whilst giving new punters the old price minus 10% to get them in the door. I am not loyal to any supplier of anything anymore and don't even participate in any reward cons offered by supermarkets or oil companies. None of them (including ISPs) have done anything to deserve customer loyalty so they won't be getting it from me.

In my line of work, Satellite uplinks, I travel to many regional areas of Victoria, SA and NSW and having a mobile phone that works is essential so my mobile contract is with Telstra.
In the uplink truck we have sims for Telstra and Optus and I can confirm that 90% of the time the Telstra coverage is far superior to Optus.
On major events in a regional area Telstra often sets up a portable hotspot van and portable tower to increase the phone bandwidth however it is only for phone coverage not data.
If you are a grey nomad or do a lot of travelling then Telstra is your best bet.
WRT changing your plan without your consent I'm pretty sure that's illegal.
Maybe a call to VCAT is in order.
I've had the call centre in Mumbai or wherever call me trying to entice me into changing plans as the old one is being phased out but for legacy customers the old plan has to remain if you don't want to "upgrade"..
It's just that the old plan is no longer available for new customers.

It is a blatant penalty for loyalty

Yep.

Meanwhile, over on the iiNet front, for nearly a week my outgoing SMTP emails have been either delayed by exactly two hours or not delivered at all. I have provided iiNet with copious examples -- especially mails to myself with headers showing exactly where the 2 hour delay is between their own mail servers -- yet they are still floundering around without fixing anything.

It took me numerous phone calls to get past the script-reading first level "help" desk to someone there allegedly a supervisor who could raise a trouble ticket for me. Until then I had to put up with the usual 'reboot your computer' nonsense and one of their geniuses telling me that I had take it up with Microsoft.

Clearly, these days the "ii" part of iiNet stands for incredibly incompetent.

Unfortunately if I leave IINET I have to leave my fibre to the house connection as well