As a lawyer, I am normally incredibly intolerant of things which deviate from standard conventions or have come into common usage due to the Internet or trends.
However, I have absolutely no problems with the use of the @ symbol. I don't think it is overused at all, unless you believe things like e-mail addresses are overused. On twitter it is used in front of a person's tweet ID to address a tweet to a particular person. Nothing wrong with "Having dinner with @Elmo @Grover" (except perhaps the fact that the world probably doesn't want to know I'm having dinner with characters from Sesame Street).
I guess the reason why I don't have a problem with this is because it does not break the convention for the use of the @ symbol. It is perfectly correct to use it as an abbreviation for 'at'. Emboldening names etc to convey the same might work on forums like this, but it doesn't work on things like SMS or twitter for obvious reasons.
The above all said, I don't believe the @ symbol should ever be used in formal drafting/writing on the basis of the rule that one should not abbreviate in such circumstances (so no abbreviated apostrophes, ampersands and @ symbols).