We use it for parents and children, patrons who have multiple cards with us (adult/teachers, child/student), and for consenting adults (married couples, caregivers). In the case of the last category, we ask the adults to sign a form stating that the other party has permission to use their card to pick up holds, etc.
We link juvenile accounts to the parent/caregiver account(s), as part of the registration process, as well as consenting adults. For the adults, there is a form that they must sign to link/unlink. When juvenile cards are updated to adult status (18 in our case), we unlink the cards. The issue we have with linking is that the patron wrongly assumes that it means they can log into one of the accounts (MyLibrary or online account in Encore/WebPAC) and see/manage all of the linked accounts. We explain regularly that it's really just on the staff side.
We also use it for parents and children and consenting adults. Additionally, we use it to link fraudulent accounts. It's been a little problematic for us, in that staff has marketed it a bit to patrons (I didn't foresee this wrinkle), rather than using it solely as a staff tool, and patrons expect some actual functionality from it. Overall, it's a positive, though.