They are pretty common. Not as common as sex links or RIR's, you can find em at most places.
We have a few silkies. Not all silkies are sweet and family friendly.
When we bought our 2 first birds, we got them expecting them to be nice. One RIR, and one Silkie, named Sophie. They were both sweet. Towards adulthood, the RIR was showing aggressive male traits, and the silkie, well "She" was sweet everyday. That is, until "She" crowed. Boy, could that chicken change overnight.
He became one of the meanest birds we ever had. He was aggressive to everybody, and pecked hard. This is just a warning that not all are family friendly.
Silkies aren't great egg layers, either. they lay small eggs, and dont lay much.
Our family doesn't believe that it's the breed that defines it's character, it's the chickens personality itself. We also had some other chickens, I forget what breed, that were said to be very mean. They were SO sweet, you'd think we got a different breed.
Good luck with any chicken you get