I tried to adopt an Aust Cattle Dog (adult dog). She was wonderful in the house, but growled and lunged at children and bikes (really anything that moved). I couldn't take her out for walks safely, and had to give her back to the shelter. It broke my heart.
They had already been to my apt and done my background check, and they asked that I consider taking another adult dog. I chose a puppy instead, as I decided the risk of an adult dog with an unknown background was just too high with my kid and the kids in my neighborhood.
My puppy is an Aust shepherd/setter mix. And maybe some collie. From the start I worked with him so he could be a therapy dog. He's wonderful and easy-going and friendly. Despite his herding and hunting breed mix, he stays with me when we go out in the woods. (now he's almost 5 years old)
I strongly believe that shelter dogs are the way to go, but be very careful with adult dogs and learn as much about them as you can before you adopt them.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson