If you look for information about it 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
This magazine is dedicated to discussions on open source software, hardware, and technology. Whether you are a developer, a tech enthusiast, or simply interested in the philosophy of open source, this is the place for you. Here you can share your knowledge, ask questions, and engage in discussions on topics such as open source programming languages, operating systems, hardware, and more. From the benefits and challenges of open source to the latest developments and trends, this category covers a wide range of topics related to open source.
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