The button should be easy pressed. It is extremely important. I know a several devices, that are just great, but they did not come into common use only because of a very inconvenient button press. When choosing a device, pay attention to it.
By pressing the button contacts must be in tight contact with a battery, in the absence of a constant contact system. This helps to prolong life of mech and to provide a minimum voltage till the next cleaning. Now mechs without constant contact still can be found at the market, but every year their market share is cut down significantly. In addition, the fewer details in the contact unit, the less losses of voltages and that is an axiom.
There are two main types of buttons: magnetic and spring. In the first case, two poles of a magnet, repelled from each other, serve as a breaker. In the second case, the button is pressed by means of spring resistance. Easy as piece of cake.