Believe I saw this on HN recently: http://haacked.com/archive/2007/08/21/i-knew-how-to-validate...
With qmail the "standard"[1] was [email protected], rather than [email protected]. I rather prefer that - but then my (user)name(s) don't contain any hyphens.
Either way I think it looks better with system usernames - [email protected] was/is usually handled as a separate form of alias look-up anyway.
[1] http://www.lifewithqmail.org/lwq.html#dot-qmail-files See section "4.1.5. extension addresses"
[ed: and I recall using tmda as an anti-spam system - it inserts an encrypted tag in the alias portion, that can be stamped with an expiry date, and tied to a sender address (eg hmac(userkey, [email protected]+01012018) => xyxyzzzzz - give [email protected] the address [email protected] - and mail to that address will be accepted from only [email protected] until 1/1/2018.
The only downsides I see are that (very few) sites still complain about the perfectly legal + symbol and some sites / bots are probably starting to reverse engineer that since it still exposes your username. But so far, I've yet to have a single site or service expose my primary mail account, so it at least helps.
Finally: this is a fantastic hack for testing your own signup flows. :)