If you look for information about it (edit: of violentmonkey) years ago you will probably find negative reviews, as it had an ambiguous privacy policy that generated many suspicions and accusations; but they fixed it a long ago, it is an excellent addon.
Opensource
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If you are talking about tampermonkey, I am sure it is a good addon. But it is no longer opensource. This post was meant for people who prefer opensource over proprietary as much as possible.
One of the things I used violentmonkey for was to redirect any reddit url to the old reddit.
Example
// ==UserScript==
// @name Old reddit redirect
// @namespace Violentmonkey Scripts
// @match https://www.reddit.com/*
// @grant none
// @version 1.0
// @author -
// @description 10/8/2022, 8:20:18 PM
// ==/UserScript==
var str = window.location.href;
window.location.replace(str.replace("www.reddit.com", "old.reddit.com"));
I wrote my first script to redirect Reddit to libreddit https://kbin.social/m/RedditMigration/t/104795/Reddit-to-Teddit-Libreddit-Redirect-Userscripts