The plugin is great, however all plugins should be developed with the wp-config.php option of WP_DEBUG to true.
Activating it with your plugin enabled produces many errors.
Can you work to resolve these errors for the next update?
e.g.:
Notice: Undefined index: validation_message in /path/wp-content/plugins/pie-register/classes/registration_form.php on line 59
Strict Standards: Non-static method PieReg_Base::pie_registration_url() should not be called statically in /path/wp-content/plugins/pie-register/pie-register.php on line 3537
Strict Standards: Non-static method PieReg_Base::pie_lostpassword_url() should not be called statically in /path/wp-content/plugins/pie-register/pie-register.php on line 3545