I used my certificate successfully today to book 2 RT flights from Dayton to Phoenix 11/7-11/11 for $442.00, taxes included. Delta.com was pricing the same flights for $560.
I called the AMEX Platinum 2-for-1 program and they offered the same price ($442 for 2) but they could only offer via American Airlines. I'm not a member of AA so I declined.
All in all, I saved a little over $100 off retail plus I get to pocket the SkyMiles for one pax (me). Plus, unlike the companion certificate, I didn't have to go to a Delta ticketing agent to complete the purchase. ^