My definition is that taxpayers benefit if the privatised company provides a materially better service and/or has a lower cost than the publicly owned version.
Telstra's service has improved due to changing technology, but it's reasonably safe to assume this would have happened regardless of ownership. Prices have gone up, to provide shareholder profits, and customer service has either stayed the same or gone backwards. Fail.
The price of electricity has skyrocketed as a direct result of privatisation, and there hasn't been any real improvement in the service provided. If anything, it's deteriorated (ref: SA state-wide blackout). Electricity is an essential service, which is by definition something that governments should be providing, rather than having private companies gouging the public.
I have no position on the AWB, Iraq scandal aside.