The -300 "stop gap" has been stopping a pretty big gap (20 years now!).
It's a very capable aircraft - built like a brick s%^thouse - although the fuel burn is roughly 3% more than a -400. That's a lot of bucks with oil at US$56 a barrel. However, the aircraft are now all fully depreciated and are configured for 450 odd pax.
I believe they have the lowest cost per ASK on sectors of between 6 and 9 hours in the QF fleet.