I have a Jack and he is one of the best dogs I have ever knowen. He is a medium energy dog, but he is very well behaved. He is good with kids, horses, cats, other dogs and just about anything (other than gophers) And he is a stud dog. He's only issue is that he will jump on people when they come into the yard (I'm still working on that) and he is not even a year old yet. I think it all depends on how you raise a dog.
Here are some pictures of my guy!