Ok, my turn to chime in! Bare with me, I have a bad cold and my medicine makes me lose train of thought very quickly. So here goes...
I bought a puppy back in November, we got him when he was about 8 weeks old. He is now 8 months old. We did have problems when he was younger with him chasing my chickens, guineas, ducks.... and so on. He killed 2 chickens, and 1 guinea. But I didnt give up on him. He was a herding dog from the time he was born. When we would walk, he would always cut me off. Making me stop. HAHA. With my kids, he would nip their heels (not hard) but to make them STOP running in the house (which they knew they shouldnt be doing anyways). Aussies are born herders. If you train them and correct them when they are wrong, you can make them "flock protectors". It's all in the training. I took him with me to the pens everyday and let him know it's not ok to even THINK about chasing them. He would lunge, and on his leash he would get a firm, fast tug, and a loud firm tone NO! Aussies are EXTREMEMLY smart. He knew how to shake @ 10 weeks old. He has then learned to fetch, sit, shake (i say high 5 sometimes and put my hand up high, and he does it), he knows COME, stay, lay down, and GIT (we use git to tell him to get away from something, or when we are busy and he is in our face.) They do demand alot of attention. But he is a protector. He is VERY protective of his human family, as well as his poultry family. Right now he is learning that my bunnys are pets and we dont stare at them and get excited. 
    Back to the protectiveness... since he is protective, anything, or anyone that comes in the yard that he doesnt know, he barks his fool head off. (I like it, he warns us). We had gone on an overnight trip to my moms to celebrate mothers day... when we came home, there was blood all over the steps, the porch, and we had no idea what had happened. Then I called Spike. He was covered from the tip of his head to his chest to his front legs. I immediately rushed him in the house and looked him over. He had a quarter sized gash in his left leg. He must have fought HEAD ON with a Javelina. I think the tuft on the pig sliced him clean. He fought for something, I dont know what. But he won. There were no other animals harmed and everyone was in their place.
   Another time, Spike saved our life, was when a rattlesnake was under our front porch. We have a fenced in play area especially for the kids and they were on top of the porch playing. I thought everything was fine. Then Spike started barking and growling, and "talking". I went flying out onto the porch and looked around. I expected a skunk. He wouldnt leave a certain part of the porch. So i kneeled down, and looked under the spot where he was most concerned. There it was. A rattlesnake. Where the kids could have been playing. My goodness. He saved the kids life. The hospitals around here are 30 minutes away. 
   And The other day, my husbands friends was playfully telling our daughter to get in the house and get shoes on, and he smacked her on the butt a few times. Spike immediately ran towards them, and got between them and barked. I had to tell him "come" and that is was ok. He wouldnt bite any stranger but he does protect. Cars, he will herd them, and try to nip the tires. (Since we learned that, he knows not to nip the tires, as he could get pulled under and killed). He does chase the kids in the power wheels, but isnt aggressive about it. He is a very very fun dog, he plays in the snow with us, loves to play fetch, and loves to dig! He did come to us with food aggression, which was corrected right away. Now we are struggling with a little aggression when the kids tug on him. He bit my daughter on top of her head (when i wasnt around). So.... now he is learning that he cannot bit for any reason, whether i am there or NOT! 
Even though he is 8 months old, that dog has been through alot. And I have yet to give up on him. It takes hard work and consistency. But overall , you can make your aussie do whatever you would like him/her to.....
edited to add some pictures of my hero dog.... 
Spike and my daughter
Spike herding me with the dead snake (head cut off)
and heres an older picture of him (hes a great listener)
I have tons of more pictures of him, if you want to see him. LOL. He has one blue eye and one brown eye.