I use Google Apps for Domains for my primary email (email@example.com). G+ is not available for Google Apps accounts yet. So I use G+ as firstname.lastname@example.org.
Unlike most other social networking systems, there doesn't seem to be a way to tell G+ that I have other email addresses (such as email@example.com) which people might try to share things with.
So now I get email notifications to my firstname.lastname@example.org account telling me that someone has shared something with me, and including a big red button inviting me to check out G+. But of course when I click it it tells me that G+ isn't available for me yet.
NO option to say "hey, I already use G+ from another account."
And of course NO option to say "quit sending me fucking notifications to this account."
I'm rapidly going from profound ambivalence to active annoyance at G+. Good job, Google!