I have an aussie. She is now about 1 1/2 years old, I've had her since she was a pup (she is still a puppy if you ask me). She has ALOT of energy..and I may be understating. This dog must have some room to run or some way to work off all the energy.
I love my aussie "Cookie" but I feel she would have been better as an only dog, she constantly needs to care for everyone in the home, including the other dogs, cats, us, and needs to herd everything. She is much more calm when she is with me on a one on one situation and not distracted with the need to care for everyone and the continual check ups she feel compelled to do. It's hard for her to "relax". When she does just plop over with her head in my lap,,,oh she is such a love.
She isn't the largest in our home but she has the largest personality and is definately the leader of the pack. She is such a smart dog, she need to be stimulated mentally and physically. She is a handful, and most aussies I've ever known are handfuls.
People will define what a handful is differently. I have to be pretty constantly up on what Cookie is up to or into and while she has a beautiful loving personality she isn't necessarily inclined to "please" me. My german shep will go out of his way to get my approval and while Cookie enjoys my approval, she isn't driven to achieve it.
If you get an Aussie, my advice is take the time for socialization and training. I would offer this advice on any breed but of any dog I have ever owned or known this breed has been the most challenging.