I got the experience the NBN in action today as my mum is now connected to it. My mum got a 100Mbps connection.
All I can say is that it is FAST. We downloaded an entire HD movie to her T-Box in under 10 minutes (The Avengers). Pages for AP basically opened in full as quick as you could click them. I was testing it out and jumping from link to link at one stage and it was amazing. Even attaching photos to upload. As soon as I clicked they were uploaded and ready.
One thing we did notice is that now her phone line goes through the NBN, if she is on the phone, and I call, I do not get an engaged signal, I got a funny little beep (lasts about 2 seconds) and then disconnected. And the tone you get for it ringing when you call it, is different as well. It still sounds like a ring tone, it's just different.
Certainly speed wise, it is an amazing increase on ADSL.