They are noisy. While the normal adult magpie's song is one of the most pleasing and characteristic sounds of the Australian bush, as the Wiki article says:
Fledgling and juvenile magpies emit a repeated short and loud (80 dB), high-pitched (8 kHz) begging call.(85 dB over time needs hearing protection for humans.) I can see why magpie parents go off their offspring, but with a beak like that, I'm surprised we don't find more young magpies 'Stabbed in the garden with a mystery sharp object'.
Of course to most Australians, magpies are a more personal threat, magpie swooping in the breeding season is something that can be a real challenge. It's worth noting that the Australian magpie (Cracticus tibicen) has nothing to do with the European bird, and is yet another example of a creature getting named by homesick immigrants. A good article on magpie swooping and the background here. Mostly, though they are no trouble - a lot less to us than we are to them, and they are a great bird to have around.
Just avoid the juveniles.