I've got one. He's smart, but not very scary. He isn't quite full grown, and we don't do dog sports. Instead, he has a slightly older Australian Shepherd as a workout buddy. A few hours each day running
around our one acre, wrestling, jumping over things, and playing 'bitey face', and add in some help from our pound mutt, and he's ready for his nap!
While still VERY young: 
Gotta be careful who you piss off!
There are exceptions in every breed. I've known a couple of calm BCs . . . one was an agility champion, but his owner was a professional handler and looked all over the country to find him. He was a "switch" dog - calm as could be until you went to the line.
I have an border collie aussie shepard mix. Intellegent and fun dog
Our shnauser was dumber than a box of rocks.
OUr yorkie was another people, except brighter than most.
Our standard cockerspanials ...this was forty years ago, were so intelligent they were known in the county.
