The small box that appears (the one that currently shows basic stats: when they joined, how many posts they have written, etc.) contains also a textarea field
Whatever I write in that field persists between my logins, until I edit it again (if ever)
The contents of that field are only visible to me
Motivation: As I read LW more, I'm starting to attribute expertise in some subjects to some users. And conversely, I'm noticing specific biases in other users. I would like to privately keep note of that, so that I can more precisely trust/distrust what they write in the future.
I believe [this](https://www.lesswrong.com/posts/ftbtZy9dBmYC5StQz/lesswrong-v2-0-anti-kibitzer-hides-comment-authors-and-vote) is what you are referring to. I have the same preference as you and I use Marcello's script.
I see both privacy issues (without pretty strong guarantees, at least site admins will be able to see my notes) and personal-bias issues (it's likely to make me over-weight opinions that I put into this field). I would likely not use it very much, but I don't object to others doing so. I wonder if keeping a separate notepad or app would get you 80% of the way there.
I also wonder if tagging of users (public, and limited complexity of comment) would be more useful or annoying.
I'd like to use this feature, especially to keep track if I meet a user in the walled garden or IRL but need consistency to remember which user they are. This is a common feature in video games and without it I would have no idea who most of my friends in League of Legends are.
I wouldn't be that worried about privacy for the notes, since I'd expect few of them to contain sensitive information, though they might contain some awkward information.
Feature use scenario:
Motivation: As I read LW more, I'm starting to attribute expertise in some subjects to some users. And conversely, I'm noticing specific biases in other users. I would like to privately keep note of that, so that I can more precisely trust/distrust what they write in the future.