I find that this easily filters out machine generated addresses, always works on forms, and if spambots ever get wise to it, it'd be difficult for them to add in the dot at the correct place, whereas taking out the stuff after the plus would be quite easy
I don't use gmail, but used to use <prefix><dash>username (prefix-username@) with qmail - and at some point had to have email at a site that used Exim with standard setup - that allowed <prefix><plus>username (prefix+user@) -- but not with a dash (it is possible to configure Exim to do this - but with qmail it is standard).
That's how I discovered that a lot of sites erroneously filter out + from the username part.
Now host my own email with Exim - but rather than use a prefix, I've just set up Exim to route all users at all subdomains to my inbox, so I can use adresses like:
site@s.example.com (s for spam). So far I that have worked fine.
I get very strange looks whenever I give out my email address in person. But you're right; it's an excellent way to track who gives out my email and it's an easy way to block spam that gets through the filter.