Where do you live? Where do you travel?
We get $50 companion certificates with our Alaska Airlines Bank of America cards. They are great for very expensive tickets to places we really need or want to visit, like Anchorage for the Summer Solstice. @:-) The companion on these certs gets miles and can be upgraded with miles or on the day of travel with money. We have also used these certificates to buy two first class seats, which ended up being cheaper than two coach seats without the cert. We have six of these accounts and get six certs a year and these certs can be used by anyone.
We also get Companion Certs from our Chase UA Platinum cards. These are FREE certs, of course they let you pay the taxes, but the companion gets no miles and can't upgrade. These are great for taking along our kids to places where they want to join us but don't want to buy a ticket. For instance, we are taking our son and his girlfriend along
to see our daughter sing at the Andy Warhol Museum on my birthday. These certs are only good for Zone 1 tickets, that is Canada, Mexico and the continental USA. We only have two of these cards. For theses certs, the owner of the card must be the paid traveler.
All in all, companion certs are a great deal if you have lots of friends and family who like to travel with you and aren't too fussy about traveling in coach.