I've been with Optus, 3, Vivid Wireless and Vodaphone for Wireless Broadband and they have all let me down with a network that just wasn't reliable enough.
I recently signed up with Telstra at the Hillarys store and I have to say that I was very very impressed. Zoe was the sales girl who served me and she was one of the best salespeople that I have ever dealt with. She explained every-thing clearly and didn't try to sell me anything I wasn't after. She even went as far as to put the sim card into my new ultimate modem without me asking her to and set it up so all I had to do was take it home and connect to the net. I have to say too that I am loving the speeds I am getting with Telstra and the reliability is very solid. I've got a good feeling this time I am going to be happy.
The reason why I am giving them a 4 out of 5 for service is because technical support is not so great. They kept putting me through to Bigpond instead of Telstra and all i wanted was my account number. In the end, I gave up but I managed to contact a good sale-rep on Whirlpool forums who gave me the info I needed.

For the record, these are the experiences I have had with the competition.

Optus - The speeds are great when its working but the network is just too unreliable some of the time.
Vodaphone - As above but a little bit worse. Some nights I couldnt connect at all.
Vivid - They were great for web surfing but little else. I found that Steam and Youtube were slow with them,
3 - Great to begin with but slowly got worse over time.

Approximate cost: $39 per month

Walked into the shop and there wasn't enough staff to serve. Then had to wait for someone to finish processing an order. Other members then became available but didn't come to assist. Ended up walking out.