I used to have a bunch of those foil getter ones. Sellers used to market them as “Silver Dragons” because they’re a bunch of cornballs. They’re not drastically different from the common Chinese 12ax7’s, but depending on the circuit and everything it’s noticeable. They’re very high gain and it’s pretty much the same character as the other Chinese but not as muddy in the low mids/bass IME.
So if you like what chinese 12ax7’s do but wish they were a little clearer, you will probably be happy. In some amps you probably won’t be able to tell a difference at all.