I've read regarding Australian field guides recently after completing my own foray into the available options. I'm a relative beginner to birdwatching and hope to refer back to this thread in due course and compare my experiences down the line with those of now.
This summary will therefore appeal to those who are relatively new to the scene and who won't have the luxury of experience to guide them in their choice of a field guide.
Having said all that my first purchase of field guide was Moorcombe's; Out of the 4 major ones, it seemed to have a good balance between text and picture detail. The little identifying guides near the pictures were I felt when buying the book going to be of great benefit.
After using the book for a month or so I grew disappointed with it. The pictures were really nowhere up to scratch. I had borrowed a Pizzey and Knight from the local library and a female figbird I'd observed had still left doubts in my mind about whether it wasn't an olive backed oriole through Moorcombe's guide. P & K set things straight - the instant I saw the picture I knew it had been a figbird. The picture was that much better.
To a beginner and I'm sure a seasoned veteran, that is important. The little detail descriptions in Moorcombe I felt were kind of covering up for the lack of precision in the pictures themselves rather than helping ID.
So I bought Pizzey and Knight to supplement Moorcombe. The idea was that I'd use Moorcombe in the field and P&K would sort out any doubts back home. I tried this for a while, but Moorcombe just wasn't doing it for me in the field. There were several times when I could go through the whole book and not find even a reasonable likeness to the bird in question.
When browsing the next time at a bookshop I saw Simpson & Day, a book I'd overlooked since it seemed to be smaller in size. The edition I browsed was the 7th edition printed in 2004 and I was actually amazed by the quality of the pictures. P&K had seemed the best previously, but I think that's due to the illusion that higher quality paper and glossy printing creates. The pictures in P&K actually have a yellow/brown cast (maybe the artist has cataracts?).
The proof of the pudding is in the eating though. I bought Simpson & Day also and decided I'd take it out and see how it went. So far it's the best by far - the pictures have left little doubt and in the case of the superb fairy-wren female the picture was so good I could say with confidence that is what I saw; examining the picture in Moorcombe I was disappointed it did not provide the same certainty.
They all have their ups/downs but I have found Simpson & Day to be the best field guide by far for a beginner. It may not have the birds aligned with the text but I almost always just browse the pictures looking for the bird and it is easy enough to look for it in the opposite page. The fact it displays birds in 'jizzy' ways; postures etc. is a big advantage it has over P&K where everything points left in a standard upright pose.
So if you are a beginner then Simpson & Day 7th Edition comes highly recommended. If you can get all three, then I recommend that also; but don't miss Simpson & Day which is a real gem and the pictures I feel are the best overall.