I accept them, but they still must book online with a credit card to back it up (but I don't charge a deposit.)
There are two kinds, those that they buy directly and those that they buy at Costco. The ones from Costco you never take! Those can only be used on B&B's website and have to be booked through them, unless they were purchased earlier than June 2009. (I have yet to see one of these) and they are limited to 2 cards per stay.
The other ones, you reed through the B&B website after you log in. From Home Base you go to Cards/Certificates at the top and choose Accept Cards/Certificates. On the left hand side you click on Redeem Cards/Certs.
To be honest, I get so few of them that it isn't worth worrying about. And for the most part, it only covers part of the costs. And my summers are so tight that I usually get them only in the slow season.
I have, on occassion had someone come up to the door with no reservation and ask me if we accept them. When I say that I do they ask if I have availability immediately... which I never have had. (As I said, summers... I'm full, you have to reserve well in advance.)