Is it worth paying extra to fly Qantas the entire way? Or are the US airlines tolerable?
With high competition on the Sydney to LA routes, prices there are relatively low.
I would highly recommend looking at getting a flight Sydney to LA, then making your connection on another flight from LA to NYC. Should be much cheaper than flying the full length on Quantas.
This little trick saves more than 25% of the price on flights from Canada (Toronto, Montreal, Ottawa) to Sydney, so I would be willing to bet the same would go for Sydney to NYC. almost 10 years ago
V Australia and VirginAmerica are the two best airlines I've flown; way better than Qantas or other US carriers. And they're cheap at the moment, and you can book it on a single ticket with V Australia:
http://adioso.com/us/los-angeles-to-nyc.html almost 10 years ago