I frequently babysit two Jack Russels and have been since they were puppies.
They are fairly smart, independant thinking dogs.
Keep in mind they ARE a Terrier - which means instinct usually overcomes command. All though, there are some owners who put time, energy into their dogs fully and end up with amazingly well trained Jacks.
If you get a wire haired Jack - they DO require clipping to keep "neat".
I suggest teaching at a young age with the Jacks as well as socialize!!