My experiences:A couple posts in the con GMing thread got me thinking about the X Card. I have never seen it used in actual play, or used it myself. I run a decent amount of con games and have run games at stores and other public situations, and it has never come up.
Have you used it? Have you had it used in a game you were running or playing? If so, what were the circumstances (be kind to the people involved, please, and don't out anyone for their preferences, please). I am honestly curious what it looks like "in the wild" rather than simply as a theoretical thing in a book.
1. Didn't have it and needed it: One player got loot-gobliny, to the detriment of the play experience, just after a discussion about "no PvP." It was an honest mistake, and was handled well by all parties involved. But it was an issue that the X-card would have made a more nuanced or perhaps "graceful" way to start the discussion, as opposed to what happened: one player had to just build up the confidence and say, "Look, this situation happened and it was frankly problematic, and undermined your words as the GM." It could have gone sideways if anyone got defensive, but we didn't, so it was fine, but I really feel the card would have helped open the discussion better.
2. Had it...but never needed it: I've since always had it available, and never once needed it. I've since found that running for friends hasn't required it because we are very mature adults that do a great job of describing the "Movie/TV Rating" and tone of our games during session 0 (which usually is just 10 minutes of us BSing before we jump into session 1). But at all my public games, I've definitely felt more confident having it available but not needing it because I start sessions with strangers by outlining the Rating+Tone in similar terms as my home games. "It's like Lord of the Rings; you'll get some torturous backstories, and orcs might lose their heads." Or, "It's like Daredevil: there's gonna some really graphic violence and underworld-type situations, but we'll completely avoid sexual situations (esp. anything unwanted) and fade to black for any heavy intimacy stuff." Notably, I won't run public games that are anything past fairly tame PG-13 standards unless it's a specific event: I ran a Halloween event session that was requested to dive deep into gore/zombie stuff, so that's the only time I ever went into R territory.