Thank you lain, you're right. Why would I use the @ syntax if I can just write the user's username by hand. I've overthought that... or maybe not. What do you think about if the syntax would work in this case also? My thoughts: typing the username through @ is faster than doing it manually and the auto completion ensures that I won't make a typo in the username. Also, I think it's more pleasant for the receiver if he/she sees his/her username in the answer / comment.

Should the autocompletion work when I reply to an user answer and I refer to the user, who posted the answer? i.e. here: superuser.com/questions/401933/…