Actually I do not agree with either of the opinions that Australia are the best/worst fielding sides. They are a good fielding side that saves a lot of runs but not the best. As aditya123 mentioned, they do tend to drop a lot of catches, and it seems that they are not even criticized too much. For example, the other day I just turned on the TV (Aus vs Pak, 8th match) and Damien Martyn dropped a pretty much sitter at gully. No one really grilled him for that.
Australia is not flawless in the fielding department--in fact it is their major weakness, compared to their other skills in cricket.