My daughter got them for us for Birthday and Christmas presents. We used them problem free. I seem to remember that you have to tell the front desk about them at check-in. We did that, and they seemd to not care. Presented them again at check-out, and got them applied right away, and received an updated bill for the remainder on the credit card.
If you plan on express check-out, I probably would not use them. I also seem to remember that they were not good for everything, i.e. restaurants, gift shops, etc, but I'm really not sure about which.

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Have a nice day flying United!
ChgoBob