Good thread summing up the contenders pretty well. I'll try to do a summary:
Simpson & Day. Excellent illustrations in the main, sometimes regarded as the best of all, very terse, often unhelpful text lacking in flavour. Up-to-date and accurate.
Moorcombe: the best single-volume guide for the visiting birder or anyone learning Australian birds: good illustrations, brilliant layout that lets you see more birds at a glance and find things faster, informative, readable text, detailed ID tips when you need them.
Pizzey & Knight. Excellent illustrations, superb text with even more flavour than Morecombe, really useful detail about distribution and etc in the entries. Second only to Morecome as an introductory guide, not as space-efficient or time-efficient if you are trying to ID something unfamiliar. Regarded as the all-round best Australian guide for those who already know the local birds, and pretty good for visitors too.
Slater, Slater & Slater. Very dated now, but pocket-sized and quite cheap. The most popular beginners field-guide, I suspect. Infuriatingly difficult to find things in because it follows Century of the Fruitbat taxonomic order - drives me nuts! But very helpful too in that it doesn't just say "a small, olive-breasted whistler" or "a large whistler with distinctive golden plumage", it stats the section on whistlers with a few words about what a whistler actually is. None of the other guides do this and it's invaluable if you are just starting out in this country.
If this is your first trip to Oz, Morecombe is my recommendation, but all four are good. Please yourself.