Another point - Tried it on Delta e-gift cards. As others have mentioned, delta GCs are now displayed as a GC on the charge line, and the CSR immediately said the credit does not apply to GCs. Earlier this year prior to the change in reporting the GCs, like sept, I was able to get $150 worth of delta GCs without difficulty.
Although i think still worthwhile for the club certs and gold status, definitely changes the value of the card for those of us who don't usually purchase incidentals.