No, you don't have to register (or re-register if you were previously a Demand 5 user) to watch clips or full episodes on the new Channel 5 site.
Interacting with the site is smoother if you do register. Commenting on episodes or web pages is quicker if you are registered and logged in.
Commenting when logged in means you can add your comment directly:
But if you're not logged in, you have to provide more information before submitting your comment: