No single guide to an entire continent could ever be comprehensive, but the Lonely Planet guide has a good stab at it.
The guide is well presented and well organised, being divided up by state. City and Town sections are presented in the standard Lonely Planet way, with an introduction, maps, lists of places to sleep, shop, eat, drink etc. For me the standout feature is that the guide supplies a web address (where there is one) for every listed establishment, museum, tour company etc. This provides a great starting point to find out if what they suggest might really be for you or not, and to plan your trip. Crucially all of the information that I have relied on is up to date.
Having also looked at the Lonely Planet East Coast, Queensland and Sydney guidebooks I would say that there is enough information in the Australia guide book to preclude purchasing a more specific one.
The only major problem, as I believe with all Lonely Planet books, are the awful town and city maps. While they are just suitable for establishing if a bar you're interested in is anywhere near your hostel, they don't really provide a means of getting you there with much chance of success. The tiny versions of the public transport networks for Sydney and Melbourne are a nice idea, but not practical unless you were to go to the effort of making an enlarged photocopy version. In contrast however, the larger scale maps are quite deceent and fairly useful.
In conculsion I was extremely pleased with this guide, which made an excellent starting point for deciding what to see do and experience in Australia.