I had one hell of a time using mine. After confirming the $5 value, I wanted to use it toward a Kingdom Hearts III preorder. The cashier tried unsuccessfully for a couple minutes to use the certificate, then called a manager over, who after another few minutes noticed that it wasn't working because it's only supposed to be used for *in-stock* items, which a preorder is obviously not.
There wasn't much else I wanted to buy, so I decided to get a $10 PSN card... which was the only denomination that was out of stock. The manager and I looked all over the store and didn't see a single one.
I settled for a $20 card and was ready to check out.. but I wasn't getting my GCU discount. The manager mentioned GCU being finicky on preorders, and had to take another minute to manually adjust the price. Altogether it took at least fifteen minutes, probably more.
...And now I'm concerned about whether I'll get the $10 reward for preordering the game, since I had to jump through so many hoops to get it.
GCU comes off on pre-orders when you go pick it up, not before.