I've been working on something similar to this in my free time; yours is very nice :D
The only thing that I looked for that it doesn't seem that you've got right now is the ability for me to see the sites that I have registered - it'd be nice to be able to enter my email address and have the site tell me that you're monitoring x, y and z for me.
If you register a site you'll get an email with a link to a settings page which lists all your subscriptions and offer one-click unsubscribe. For now I only allow one site per email though. The plan is to offer several sites per email and lower poll intervals for a small subscription fee in the future if there is any demand.
I poll every 15 minutes. I should probably mention that somewhere for the technically minded.
You may want to change the verbiage of the email - it looked like it was just an unsubscribe link to me. I wouldn't have clicked it at all except for the fact that you just said it would take me to a settings page.
(Right now it just says: "To unsubscribe from this service, go to: <link elided>")
I would suggest that you don't tie the ability to monitor multiple sites with one email to a subscription fee - since that would be so trivial for people to get around for free.
The only thing that I looked for that it doesn't seem that you've got right now is the ability for me to see the sites that I have registered - it'd be nice to be able to enter my email address and have the site tell me that you're monitoring x, y and z for me.
How frequently are you polling?