First of all a big thank you for the great plugin - it provides exactly the funcionality I'm looking for.
Anyway, a small "heads up": I upgraded to WordPress 3.5 today and while checking all the plugins I found a warning message in the "Subscribers" panel of your plugin. The full error code is
Warning: Missing argument 2 for wpdb::prepare(), called in <PATH_REMOVED>/wp-content/plugins/newsletter/users/index.php on line 106 and defined in <PATH_REMOVED>/wp-includes/wp-db.php on line 990
That's not a real problem, this check has been inserted by the WP-Team to reduce vulnerabilities against SQL injections. The whole story is described in Andrew Nacin's Blog Post.
Otherwise I'm very happy with your plugin, thanks so much for providing (and maintaining) it!