I received and installed the firm (gray) pads on my Filco with cherry browns.
It definitely changes the feel of the keys, and it's going to take a little getting used to. I'm not sure if the length of the keystroke has been reduced (looking at two keys side by side, one with a pad and the other without, I don't see any difference in their height when I press them both down), but there's definitely more resistance near the end of the key stroke that makes it feel like the keys aren't going down as far while I'm typing. It does almost give it a sort of a rubber dome feel, so if you can't stand the feel of rubber domes, these pads may not be for you. I need to spend some more time with them before I make up my mind on whether I like the feel of them or not.
As far as its stated purpose goes, it definitely does the job it sets out to do. My keyboard is MUCH quieter now than it was before. The keys make very little sound now on the downstroke (and what sound there is is more of a dull "thump" than a sharp "clack"), so the upstroke is now by far the largest source of sound coming from the keyboard. If only Filco had used thicker key caps to help reduce the upstroke noise, this would be a truly silent keyboard. As it is, it's still quieter than even some rubber domes I've used.
I might look into those o-rings and see how they compare to the pads. I'd also like to hear some suggestions for which ones to try from people who have used them.