Rated 5 out of 5 stars

There are more downloads at the developers' site:
http://enigmail.mozdev.org/download/index.php

NB, if you have 64bit or AMD64 Linux, you need the 64bit extension, only available at their site.

Great extension! It's unobtrusive and the initial setup is just this:
1) install ThunderBird (I use Ubuntu, and it's not the default email client)
2) install Enigmail extension
3) Follow instructions http://enigmail.mozdev.org/documentation/quickstart-ch2.php

Note that for users of other OS's, you may have additional steps. Step 3 is basically just a howto and best-practice on how to use the Enigmail Wizard.

BTW, I don't know why it turns off HTML email as the default... I have tested it with PGP/MIME on HTML emails and it seems to work fine...

This review is for a previous version of the add-on (1.0).