a jack x chihuahua could be a great dog or a horrible one depending on what your looking for...
but from that kind of mix id expect LOTS of noise, both breeds tend to be talkers.
expect HIGH energy, chihauhas get a rap for being lap dogs as it is, but in all honesty they are very high energey for a little dog, and jacks, well jack are on the go!
prey drive, chihuhauas are mini hunters and while often fine with their own breed, often dont get along with strange dogs, jacks are also drone to dog agression with strange dogs...it would definatly be a puppy id MAJORLY socilize, and go to training classes with.
id also expect a dog whos very connected to their person.
reaserch a jack/parsons and a chihuahua, take the WORSE traits form both, combine them together and if you can handle those worst traits in one dog youll be set lol