New kid here, in fact I joined just to post on this topic.
I have a 15 month old Griffon male who I adore. Most of the things mentioned
by the people who have owned them rings true. I can speak for my boy and his dad, but have met few other griffs up here so my experiences are also relative, so please dont take them as fact
Luke has a wonderful personality, he is great in the house when supervised he has a great off switch in his kennel and will zonk out while we are out of the house without him. Out in the field or pasture he is out hunting, his desire to hunt is higher then his desire to please me.
They are a much different personality then any herding or working dog, they are more like a scent hound in their drive once Luke gets a scent for a bird he is after it and it takes a while to call him off of it. Like I said, high prey drive, if my horse are running he is after them trying to get them to chase him and play with him.
As for the hands on when you are working them this can be true, but Luke has a great whoa, down and sit. I can put him on a sit and go do something out of sight for five minutes and come back to release him. they are SMART, the first week we had him Luke knew how to shake, bow and play dead.
They NEED something to expend their energy, so if you live in an apartment or small property know that you will have to take them out on a long working walk every single day, if I skip a hunting session or hike with luke once I know it for the rest of the week. this is ontop of yard work, our obedience or while I change water.
Luke loves to dig, I fill in many deep holes every weekend because he gets the smell of a vole or something and can not let it go until he has eaten it, most breeders will say that they are diggers.
As for chewing, Luke loves to chew I bought him an Orka jack toy that is his favorite, but he will carry around anything he finds to be useful in getting our attention. He does not do well alone in the car for more then 3 minutes, he tore the headliner off the truck and the sunvisor off the van.
They are long lived and relatively healthy, Luke's dad was 15 when he sired him, and still jumping 4 feet in the air. You need to make sure their elbows and hips are clear, as well as the genetic history regarding their eyes.
Talk with 7 or 8 different breeders and see if they think the breed is right for you.