FastMail aliases are great. I've used some for sites I'm more likely to want to cut off access to me, for instance. Whereas address+site@gmail.com gives you sortability, a true alias gives you the easy ability to just shut a site out of access to you.
Another issue I had was that my main email address was already in Google and Microsoft's systems for a couple reasons, and it wouldn't let me set it properly when changing my email address everywhere. So I have google@ and microsoft@ aliases just to work around their account management quirks.
