I've bought a few items from US sellers participating in the GSP. Each time it has got here in about a week and a half (a few days to Pitney Bowes and then a week via FedEx to Aus) so I definitely wouldn't accuse it of being slow.

They don't collect customs duty or GST unless required by Australian law (if the shipment is over $1000).

Obviously there will be exceptions but everything I have bought has been WAY cheaper than the comparable USPS service. I recently paid $39 for something that would have cost a minimum of $60 through USPS and it got here in half the time. I'm more than happy to keep using it, in fact I favour US sellers that offer it.