Not the Jacks i've been around.
It's a stereotype that haunts the dog because of the movie industry.
The Mask made the Jack famous for being hyper and agressive.
I have met maybe 40+ Jacks and I have never come across the typical "Pull at your pant leg and growl" type. They are very social dogs, they love their owners and the people of their everyday life more then their own. My dog loves meeting new people and craves attention.
They are playful dogs, full of energy. No way they are vicious. The way they have been bred over the hundreds of years is to respect people and always do what it takes to make them happy.
Most people think rotweilers are vicious and mean. I was shocked when I went to the park and met 4 or 5 one day and they were the smartest ones I have ever met.
It's all up to the owner how their dogs turn out. The Jack isn't for a n00b owner, if a n00b owner buys a Jack, espect the dog to be demented and annoying because they never taught it to be controlled.