alasta: I would strongly advise against using Optus. I have had extremely painful experiences with slow speeds, poor coverage, and extremely awkward processes to view your data usage. I definitely wouldn't use them again.
I use all three networks on a daily basis, and while all are acceptable, I find Vodafone to be the best for coverage and data throughput, Telstra a close second and Optus lagging behind a bit. But certainly in terns of value I think Optus are the best.
This is in Melbourne tho, Sydney could be totally different.