I think that Aussies also use a bullpup (the Steyr).
The main advantage is that you can fit a longer barrel in a shorter rifle, and it is generally better balanced (more weight centered over the grip as opposed to in front of the grip). My busmaster M17S is several inches shorter than my bushmaster AR - but the bullpup has a 21.5" barrel, whereas the longer carbine has a 16" barrel. The bullpup is also balanced better, and I can shoot it one-handed, which is really difficult with a full length AR with a 20" barrel.
As some people pointed out, the downside for some is that the ejection port is too close to the shooter, making it difficult for lefty's to shoot (although some like the FAMAS allow a simple conversion to change the side of ejection) - and I guess a catastrophic failure of the firearm woudl be more severe in a bullpup. Some people also just hate the look - or simply fear change
and thus don't like them.
Personally I love them - and I would give one of my testicles for a British SA-80.