I like AS, and was able to use the upgrade certs on 3 out of the 4 flights that I've flown with them recently. The one time I didn't get upgraded, I was waiting in the regular line at the ticket counter, and another passenger walked up in the F line and purchased an upgrade to the last F seat. If only I had gotten to the counter a few minutes earlier!

Every time I've used the certificates, the ticket agents act as if they haven't seen them before. They go on to look over the certs very carefully before giving the upgrade.