I've gotten quite a few cards over the years. I find that the SEA based pilots tend to do it more often than others. Captain Bob being an example of one.

But it is a very nice touch when they show up. I've had a couple of pursers write notes as well over the years, but not in the past 1-2 years.

It's such an easy thing to do but really makes a lasting impression - especially when I'm travelling with someone who doesn't usually fly United.