Hello! I got into churning thanks to Ozbargain. I took advantage of the Commbank offer you wrote of; spent the $500 on things I would have bought anyway, and the $250 was back within a week. I've kept the card open in the hopes that I can also win the lunch with the Aus cricket team (part of the promotion is this draw), and once cancelled, I'll have a cool credit card with the test team celebrating.
The other one I signed up for was MEBank, who was doing a December promo of 'Open a check account, spend at least $10 three times, get $50 cash back". I've done the spending, now waiting for the $50.
With respect to credit scores I don't think it's that big a deal in Australia simply because unlike the US, where your score can dictate your loan interest rates, Australian loans seem to depend more on the actual amount being borrowed.