While it's true that my JRTs have at moments channeled speed demons, I know these little dogs are all heart and are capable of a dazzling ability to focus. I think a lot of people were charmed by the dogs used on Wishbone or other TV shows and ran out and got a Jack Russell without really researching whether this breed was a good fit for their family or not, and JRTs got a bit of a reputation as being difficult. This is a pity, and it's not the dogs' fault at all.
They are smart and can get up to mischief, but they are lionhearted little dogs and given the proper socialization, a JRT can be a devoted companion and fierce defender. I'll never forget the how my heart was stirred by the little Jack Russell who saved his family's children by giving his life defending them from two attacking pit bulls. Then there's the little dog in the video above named Spice who is acting as mother to two little orphans of another species. Yeah, I know JRTs aren't the only ones who do that, but I think this must be a very special little dog, just the same. Look how placid she is? I see that all the time with mine [unless there's a bunny in pack territory].
Anyway, if you just get occasional glimpses of them, you may not have the full picture. Sometimes you have to work very hard to be worthy of the nobility of your pet. I'm trying every day, and sometimes I fail, but it's important to have goals. I want to be like that thing on that email that went around - I want to be worthy of what my dogs think of me. :)
One more thing-- from the penetrating mind of the lovely LabRat I once heard a bit of wisdom about irritating dog breeds: they are what they were bred to be so it's really not fair to blame the dog for being annoying. Jack Russels were bred for the purpose of going to ground-- running down holes to hold or extract foxes, groundhogs and other critters their masters are hunting, for whatever reason. They zero in on fur-bearing critters something fierce, and woe betide the squirrel or rabbit they get hold of. JRTs were made to work, they want to work, and all working breeds need a purpose in their lives or they will be bored and will exasperate all around them. Just be sure you take into account what the purpose of the breed is you're getting and what personality traits tend to accompany that purpose, and make sure those quirks will fit with your family. In my humble opinion, if you could even consider having a family portrait without your doggie in it, then you should either have a different dog, or have no dog at all.