Costs of Conductive Silicone/Rubber Keypad vs. Mechanical Switches?
Does anyone have any experience implementing these? Are there any
  gotchas or things to watch out for?


I have tested a lot of tactile/membrane keypads in the past and the biggest issue I found was bounce time was very variable between one manufacturer's product and another. This can be annoying if interfacing with a spcific chip (like a DTMF encoder in a telephone) because you quite often got double digits when you thought you'd only pressed once.

If you have a high volume product or a product that gets a lot of keypad use I would seriously consider a lot of mechanical testing of different supplier's products if they do not have a technology that guarantees a closed resistance within a certain time period of the button being pressed. Same when releasing the switch - it can bounce then.

Some keypads I tested that had a seemingly beautiful tactle click didn't actually make contact until you pressed a little harder. Now I'm sure the inducstry has moved-on from those days back in the late 1980s but caution should still be your watchword.


  Any experience on the cost side?


They are cheaper on production costs for low to medium volume but don't ignore the time and effort into guaranteeing a good design. The main reason for using them is of course that they can be designed to have buttons in irregular positions i.e. they are easily customizable.

Not wishing to counter anything said by anyone else (@Jonk) but a good technology should give you over ten million operations. We (back then) modified a motorized hack saw like this: -



It produced a repetitive forward and backwards stroke and we used a spring/cushion to set the impact force onto the target keypad. We easily got ten million operations from quite a few but very few could meet the debounce times at end of life.