When writing informal text, the @ symbol will sometimes appeal as a quicker (and quirkier) way of typing ‘at’. But in more formal documents, this is likely to look offhand or careless. Avoid it unless you are striving for an informal effect.
I'm speaking @ the conference this Friday
In a business context, the @ will also commonly appear in handwritten notes or documents such as invoices. While an informal note is one thing, it can reduce the professional appearance of an invoice.
For the supply of 60 units @ $30 each
It really doesn’t take much more effort to write ‘at’. Save the @ sign for email addresses and social media handles.