Yes, the key-clicking action is important, but so is the shape of the keys. For optimum text entry, one of the most important features is: you should feel with your fingertips where are the key tops (the centers of the keys) prior to pressing the key. Even if a virtual keyboard had a good haptic feedback (like N9 has), it is not enough. An interesting innovation is the Tactus keyboad: key domes which are filled with a liquid when you activate the keyboard; see the picture on top of http://www.tactustechnology.com/technology.html . Here I'm not suggesting that solution for an OH keyboard, I mention it only as a reference of the correct shape of the keys, which can be applied also to HW keyboards. Nokia has applied the same shape. Their switching elements are water and dust proof domes, on which there is a flexible keymat onto which are mounted rigid, outbowing keys. Or, if the keymat it not flexible, the film around the switching dome is flexible, so that the rigid key can move freely, when you press it against the switching dome, making it click. Of course for experiments also flat keyboards can be built, but for finished products please use the best HW keyboards of smartphones as references.
I mention this, because IMHO it does not make sense to add only a flat membrane on the switching domes. But if the dome is sufficiently outbowing and the membrane soft enough, so that you can feel where the tops of the domes are, perhaps only a thin film on the keyboard might work well enough.