Have fun!
Saving yourself a single keystroke (shift-U vs. y-o-u) has much greater costs than you might realize in terms of how people perceive you, trust you, and are willing to engage with you.
I'm not trying to be prescriptive – I don't think using U vs. you is a moral issue, or makes you good or bad or smart or dumb or anything like that. But people react to things like that, even if it's not fair that they do so.
This is true. However, it has no costs in terms of whether your message will be understood as you intended it, or whether the audience will even have any doubt as to what you meant. Thus, I find your lead comment, "I really value clear and precise communication", interesting. What about the U symbol do you feel is unclear or imprecise?
On a different note, I generally see people use "u" rather than "U". U is harder to type, but it does bring out a certain parallelism with "I".
As I was reading the original comment, and before seeing any of the comments about the style, I found myself highly distracted by the "U". Wondering why someone wrote that way, what the purpose was, and so on.
Because of the fact that it significantly distracts from the message, I think these abnormal styles are insulting to the reader and so are not something I would recommend or condone in contexts where normal writing is convention.
(On IRC or in instant messages/Slack, the style is different and it's OK. But you can see what the style is on HN by reading comments on this page.)
I'd be interested to know how well people with dyslexia or with English as a second language cope with it.
Think about it for a minute, and you'll see that (1) it's an extremely common word, so anyone with any experience with it at all is likely to have quite a lot of experience with it, and (2) there are no other candidates for what "u" could mean. It might pose problems for foreigners living abroad whose only method of determining what a word means is looking it up in a dictionary; it poses no problems for anyone talking to the person using it (even pretty difficult things can be understood if the other person is there to help you), or just living in an english-speaking country.