Hi Don,
I went there around March this year and must admit, was pleasantly surprised how good it was. I didn't think they could cram in so much, but once inside there are quite a few things to see. However, you do need to realise its only Australian fauna - don't expect to see Elephants!.
We were there for about 2-3hrs and got our monies worth.
Highlights :
Butterfly enclosure
Snakes/reptiles
Australian marsupials.
The Sydney Aquarium is next door and that too is pretty amazing. I would also put it on the agenda. Just don't go there when its our school holidays, the line-up and crowd will test you!!!
If you want an overall experience, Taronga zoo maybe the way to go and make sure you catch a ferry from Circular Quay to get there. They've got cable cars so you get a birds eye perspective of the zoo and scenery. That will make a nice day out.
btw: If I had to choose between the 1st 2, or latter, it would be tough. Both offer different experiences and you wouldn't be disappointed either way.
