I have had three different heelers. All three have had vastly different personalities.
They are incredibly smart. They can learn to do almost anything you want them to do.
My first one, and best one, would bring me the remote whenever I sat in my chair and then he would just lay by my chair until I got up. Very, very laid back. I could almost have a conversation with him. Like said earlier, they know when they have done something wrong. The thing is, though, a lot of the time they don't care...the consequence is worth it. For instance, if Gus saw a chance to grab a piece of pizza, he would do it every time. He would almost smile at me while I was whipping him for it, too.
I have a girl dog who is very active, less social. She doesn't get along with other dogs but she is very good around anyone she knows.
Now I have a pup who is kind of a mix between the two. He loves everyone and every dog and is very active.
A heeler's puppy stage, in my experience, is incredibly easy compared to labs/goldens/boxers, etc. The key is to socialize them early and start working with them early.
In my opinion, they are the best dogs there are. Incredibly loyal.