Basic functionality works well. It copies text that you select.
The problem with this (and every other auto-copy webextension I've found) is recognizing which fields *not* to copy from. For example, this extension doesn't copy from basic text fields, which is good. But it does copy from Gmail's compose field, which is bad.
A simple mitigation for this would be a user-manageable site blacklist. Alas, this extension does not have that, or any options at all.
My search for an auto-copy extension that really works right must continue ...

its ok, but the i really MUST to have the feature to deselect text after copying. Paste with middle click is good to have as well.

Doesn't work on addons.mozilla.org website. (why?)