Ive never had one but encountered a few.
They can be described as pirahas on legs.
The ones I have met have done great earth work on badgers as well as fox. They are gaining in popularity here due to growing wild boar populations and they are indeed hard enough to get pigs going and yetfamily dogs but dogs are that if you ask me.

This pic I found on the net and in my mind it reveals the nature of these dogs.