The network has good coverage but here in Melbourne I've found that sometimes the signal is good but data just doesn't seem to get through, same on my phone.
What lets it down, on the Mac anyhow, is the software. "Telstra Turbo Connection Manager" is an amateur attempt to wrap the modem's low level functions in a graphical interface. Topping up obviously uses SIMToolkit and is barely workable. It took me four attempts just to top up - kept timing out.
To create the network on the Mac is touch and go, sometimes it works but often it just gets confused. I think it remembers the index of the device it's using and several times it's tried to connect using the Bluetooth interface.
Right now, (very early morning), ping times are about 800ms, during the day it was over 2s.
Of course all this wouldn't be necessary if my iPhone still tethered..