Impressions inspire complex feelings, at least in me.
I was initially very skeptical of them, because they are almost designed to bleed from IC to OOC since they are explicitly taking IC information the other character does not have and communicating that information to the player. This is more or less exactly what it is hoped players will keep to a minimum among themselves, for reasons I think most people, if they are honest with themselves, agree with. Namely that, as Inaya says, even the most scrupulously compartmentalized players can forget where they learned a piece of information. Nevermind all the temptations to use ill gotten information to influence the outcome of a situation to one's liking that can get the better or even the most well-intentioned. I am generally in favor of trusting players to mean well, but I think this is a different sort of situation than that. It is genuinely impossible to unring a bell. No nefarious intent required. So, given all of that, I initially thought making impressions known to the target was a terrible idea.
But over the course of reading the Reddit episodes and thinking about my baggage and my tendency to project models and issues from past games onto new games, I ended up deciding that the only way a new game was going to have the opportunity to exist outside the old (bad) mold would be if the players were able to leave behind their expectations and projections and experience it as a wholly new thing. So I decided to radically science these unfamiliar and in some cases frightening deviations from the norms I had accepted. It was an interesting and useful experience. Once I got in game I started impressioning people and I didn't die. No one stabbed me or refused to play with me.
Now, I DO edit my character's thoughts for the purposes of impressions. I spent a while feeling a bit frustrated, actually, because I didn't have the luxury of being open and straightforward (for, I think, fairly obvious reasons >.>) and finding ways of saying something worth saying without showing my hand was an extra hurdle. But receiving impressions back that allowed me to feel that my frantic flailing at the keyboard has communicated any part of the character I was trying to describe, however superficially, was hugely satisfying. Is hugely satisfying. So that made the work fairly worth it.
However, as I tend to play a fairly challenging character who tends to get into mischief, I have always had an unresolved feeling of tension around how honest to be in expressing his impressions of people with whom he is in active conflict in some way or another. It's a leap of faith each time. I know that if the player I'm engaging with is feeling too identified with their character at that moment, or is just having a rough day, they might feel upset or angry about the (extremely subjective) judgment they have just received about that character. This concern, and the resulting suspicion that some reactions may be fueled by OOC emotion of this kind has created a conflict with that commitment to radical openness and trust that I made earlier. I have started to wonder, again, what value impressions really serve.
Everyone wants to get positive feedback, and everyone's sense of what is positive varies. I am delighted to get impressions that say "what a scoundrel!" from people who seem to enjoy having that type of person to bounce off of. But sometimes I find myself feeling as if I have to choose between prodding someone who doesn't seem happy and refraining from utilizing a fundamental game mechanic. I say fundamental in part because it's one of the very limited number of ways there is to earn XP in SoA. I'm obviously not obsessed with number-go-up or I would be playing a different game, but I think everyone wants to be able to advance their character in ways that jibe with their personal journey, and there are only a very limited (which I VERY much approve of) ways of doing so.
So I am fairly ambivalent about impressions currently. I suspect that personally I have perhaps had my fun with getting them and could do without the anxiety and drama at this point. While they remain an important mechanic I will continue to do them, but I will always find it stressful to have to make decisions about what to share and how often and with whom.
One last comment more aligned with the original topic (soz Inaya >.>)... having impressions be "spent" by attaching them to story developments, and only making one impression available at a time per character seems unnecessarily restrictive. My story developments do not line up with my succession of impressions neatly enough for this to make sense at all, and they often apply to more than one development. If we are keeping them could we perhaps just store the whole list and make them available for all time?