On the $59 plan from Optus. Phone is great but the Optus apps are absolutely shocking. Most aren't even apps but just browser shortcuts to the optus zoo website. Even worse, some of those apps won't allow you to connect via wifi!! Have to use 3G Optus cell connection to use them. Waste of time, waste of space.
Get rid of them as soon as you can, especially the Optus Now app which mysteriously eats up battery even though I wasn't using it at all (not even the widget).
Their version of a GPS navigation app is shocking too, considering SGS2 comes with Navigon GPS (even Google Maps navigation is better than the Optus one) there's absolutely no use for the Optus GPS navigation.
Other than the Optus bloatware, I've been extremely happy with the phone. It's got an amazing screen. 3G speeds have been decent, averages around 3-4Mbit/s in my area and goes upto 6Mbit/s in Sydney CBD.
One issue I've had though which I've seen a lot of other Aussies are having is is with Kies saying I have a firmware update even though it lists the same one as my current version, both KE7. Mine has never been updated, came as KE7, I've only CF-rooted it.
EDIT: Just connected to Kies now and the firmware update prompt has gone. So looks like they've fixed it now. Before, connecting to Kies would always bring up the prompt to update the firmware even though the version was the same.