Works as described

Does exactly what it says it will do.

It also comes with some nifty filters if you want to customize its usage:

If you add apply_filters( 'migration_notice_disable_css', '__return_true' ); to your theme’s functions.php file, the built-in CSS file is not used. You can then specify your own CSS in your theme’s style.css file (and load one less stylesheet)

If you add apply_filters( 'migration_notice_hide_frontend', '__return_true' ); to your theme’s functions.php file, the frontend notice isn’t displayed. This is useful if you want to hook it somewhere other than wp_head (ex: if using Genesis, you might want it before the post content, using genesis_before_post_content hook).