Metal valve stem covers. Cheap, look nice, why not?
I've had a set on for a while and they seemed to be fine. Last night my TPMS indicated my tires were low, which wasn't surprising given the temperature. Easy fix, go get air. Got to QuikTrip and I can't get the covers off . I went home to grab a wrench to get a better grip: still nothing. Eventually I got a couple of them off but one wouldn't fucking budge.
Now I'm at the dealer, late for work, and waiting for them to take my car back and replace the stem. Fortunately it sounds like they can reuse the TPMS, but if they can't I don't even want to know how much it wll cost. Right now they're telling me $75 for parts and labor to replace the stem.
So TLDR don't use these fucking things or you'll be late for work and out $75. At least I get to gawk at a Lime Rock Green C7 convertible...