More useful for domestic vs international, depending upon the fare you book.
Sometimes (not always) you can book 2 one-ways at the same price, allowing you to apply a GC to each one.
I bought a few last year when I knew I had a few trips coming up to apply them to, but somewhat debatable as to the "value" of the $50 savings. Some would value the lost AP points (GC purchase multiplier vs an AC multiplier on your card) at half of the savings.