I used to use my titanium spacebar on my kmac every day, but I swapped it out for PBT after a while, as the metal was sort of starting to put a dent in my thumb. More importantly, I was starting to scratch it from use, and I want to polish it before putting it back.
I have some aluminium keycaps, that I did use briefly as "esc" keys, but now they are on my qfr which is not currently in use (waiting on a frosty flake to get done, which is waiting on a new soldering iron tip to arrive)