Before you install, Greasy Fork would like you to know that this script contains antifeatures, which are things there for the script author's benefit, rather than yours.
This script contains code that will track your browsing.
The author of this script explains:
When a metacritic rating is displayed, we may store the url of the current website and the metacritic url in our database. Log files are temporarily retained by our database hoster heroku.com and contain your IP address and browser configuration.