Two months ago I bought an Apple USB Keyboard directly from my nearest Apple Center, MicroGestió. Unfortunately, the Enter key started to behave incorrectly some days ago; its movement wasn't as smooth as that of other keys and in some cases it simply didn't move. For example, it was really hard to press it when pushed from the up or left sides.
In my case, I've noticed that I tend to press the Enter key towards the top, with the force going "upwards". This made it fail many times, requiring me to hit it again, harder and in the middle. Very annoying.
So... I went to the store this morning to ask if this was covered by the warranty; I had nothing to lose. And yes it was; they replaced the entire keyboard with a shiny new one without questions. Great!
Of course, I'm talking about this specific Apple Center. I do not know about the customer service in the others, but there surely are better and worse ones. (Would like to be proved wrong ;-)