It takes too much effort to format mentions in a helpful way. When I type "@Someone", it should:

1) Keep the @ sign in the front of that mention to help you visually identify that it's a mention

2) Just have the user's first name or a nickname rather than their full name. I know you can backspace to delete a last name, but that process doesn't flow well

3) Automatically put a space after you make the mention.

I usually would like to have my mentions look like:

@Greg can you please answer your phone?

@Tiffani @Josi is Greg here today?

Thanks for picking up the phone, @Brett