I've registered, and I'll be watching with interest to see if it changes my relationship with the site.
When I originally launched the beta I didn't include this functionality, but had a friend ask for it right away and it dawned one me that it might not be that uncommon for one HN user wanting to watch the interaction another user is having regularly (e.g. following 'pg' or 'patio11' conversations).
Only because the information is public I figured "sure, why not" and added it.
Although it occurs to me that the more appropriate feature might be a "Watch this user" type of signup which I would be happy to add down the road if people really wanted it.
When you unsubscribe, the site will unsub you from every user account the email address you entered is watching, so you don't have to undo each one individually.