There are a couple of people on AO3 who are both writers themselves as well as voracious readers. They both love to leave comments on fics - J less often than W, who seems to comment on every fic she reads, but I've gotten lots of feedback from both of them.
A few months ago, J found my most obscure multi-chapter and read it, and left this amazing, long, in-depth comment, telling me how much she loved it and which parts meant most to her. I didn't see the comment until three months later, and I made sure to respond back and apologize, and told her some of the motivations behind the story.
Well, apparently, she and W are good friends, and she recommended it to W, and I just received W's comment on it. She posted as the comment the entire chat log between the two of them as she read through the fic. It was amazing, getting to see how she reacted to the story in "real time", as it were, and the things that stood out to her the most as she read it, since she discussed them with J. And, in the middle somewhere, J said it was her favorite of my stories and decided to go re-read it.
So, I'm just kind of dancing around. It's great to be appreciated, of course, but as a writer, I'm not used to getting first-hand feedback on what I do. And it was wonderful that she realized the chat log was the best comment she could leave.