My youngest DD got one at some character meal at WDW a few years ago, and older DD got one another time at a different meal. I can't recall the meals now...
My DDs were talking about our Japan trip to a CM at Surf's Up two year ago, and he was from Japan, so he was really tickled to hear two little girls raving (without any involvement from us) about his home country. He went back and brought them back signed pictures of Mickey Mouse and an autographed birthday card for my younger because we were celebrating her birthday. It felt like a special pixie dust thing more than something we could have asked for.
We also saw a young lady with Down Syndrome receive a birthday card signed by all the characters for her birthday (also at Surf's Up). Not going to lie- DH and I both got choked up seeing how happy she was and the sweetness of the characters with her. We crossed paths with her several times on our visit and saw her experiencing a lot of pixie dust. It really warmed my heart.
Back to topic, I have brought birthday cards for my girls and had the characters sign them. It makes a great keepsake for birthdays.