QANTAS will get you there and back safely, any disruption will be competently handled and the service will be at least adequate. They are certainly a better option than any of the American airlines on the route. You are very unlikely to have the sort of nightmare experience that Air India or Kuwait Airways passengers regularly write about on Skytrax.
On the LA-SYD route you have far fewer options than you would have on the LON-SYD route where Qatar Airways or Singapore airlines provide strong competition.
Go ahead and book Qantas, it's a good choice.