My trade magazine has published a journalistic review of three Dog DNA tests. You may find this video report helpful and interesting. C&EN tested one dog’s genetics and got weird results
That was an interesting video. I can see where DNA testing might help if you have a dog that resembles a pitbull (and the restrictions that are sometimes pointed towards that "breed") and you can prove via DNA that it isn't (like my son's former Catahoula). On the other hand, there's a plethora of dogs out there who's build screams "pitbull" - but the rescue has dubbed them some mythical cross between rare breeds (rolling my eyes)