I have not used them, have no interest in using them and cannot imagine ever adding yet another thing to keep track of to the mess at our table.
I mean, seriously- character sheets that vary from three to five pages (including notes, some of the players have a clipboard full), dice, minis, props, maps, modules, papers galore, which beer is mine?, where's the damn battlemat marker?, power cards, snacks, players' reminder notes to themselves (the warden has a large table tent that he puts in front of himself that says "MARK 'EM DUMMY!" because he always used to forget), etc, etc.
Add a bunch of little loose cards to this mess? Hell no. No way, never. I occasionally use index cards for initiative, but even that has proven too easy to lose bits of. No cards in my game unless the party is playing with a deck of many things, thankyouverymuch.