Ok, so obviously it needs work! I just subscribed while making the parent comment BTW, and find the service useful.
It would make a lot of sense to allow handles to be monitored by many different people; some users are super popular and I guess many people would like to "follow" them...
So you can't use it if the person you want to "follow" is already registered.
Now, I tried "pg" and it subscribed me. The result of this is that if "pg" wanted to subscribe, he wouldn't be allowed.
I tried to unsubscribe. Unsubscription is e-mail based and not handle based. i.e: You can't unsubscribe from just one user, you unsubscribe from all the users, since you're taking out the root (e-mail address).
Now I have to subscribe again to get my own replies.
PS: Been notified of your reply by HNNotify :)