Mobile devices designers often make use of ‘press and hold‘ pattern for locking the keys, muting the phone or else. So do many industrial designers. This pattern is pretty useful when you have a limited number of keys (like in the mobile phone keypad example) or you want to make sure users don’t press the button unintentionally.
One of the downsides of this pattern is that you really have to make sure you’re using the ‘press and hold’ for not so frequently used actions. Otherwise you may end up pissing the user off. “Why do I have to wait so long for my keypad to lock?!”. I.e. you can lock Nokia phones’ keypad but pressing two keys one after another while with Samsung a single key has to be pressed and held for some time which is pretty annoying.
Another thing to keep in mind is that the key that’s meant to be ‘pressed and held’ should be durable. I used the hands-free headphones for my mobile some time ago and to answer calls I had to press and hold the only button it had. By doing so and unconsciously applying extra pressure I broke it. The same happened then to my brother who had the same set and whom I actually warned beforehand.
So if you’re designing gentle stuff, avoid the ‘press and hold’ pattern. Our body obviously thinks that it has to apply some extra power when pressing things.