From what I have seen in the breed, they are like any other breed that gains in popularity. The back yard bred/puppymill strains tend to be neurotic on the aggressive/destructo side, and the well bred strains tend to be typical terriers. Highly active, drivey, intelligent dogs that need a lot of interaction, training, and handling from their owners.
If you were to get one, I would go with a responsible breeder and meet the parents if possible to find out if they are sane or not. It would be a huge plus if the parents had some certifications, like CGC, TT, or TDI to prove their mental stability, but then that goes for any dog. Or you could go through a rescue, and adopt an adult that has been assessed and found to be stable.