This script makes YouTube automatically redirect you to your Subscription page if signed in on the website. It also allows you to visit the What to Watch page if you browse to it from the Subscription page. This applies to every link that takes you to the main page, including the "What to Watch" link and the YouTube logo. So if you're, say, on a video page you need to click What to Watch twice to get to that page.
It should be fairly quick, and should only run on the main page of YouTube (www.youtube.com). Both HTTP and HTTPS should be supported and it seems to work as intended in both Firefox and Chrome.
The script checks if the cookie "SID" exists to determine whether or not you're logged in. I'm unfamiliar with the purpose of this cookie, but my best bet would be that it includes the SESSION ID YouTube uses on the server. No data is collected or transferred at any point during the execution of the script.