[chrispadfield]

of course it is up to you but useing user groups destroys any previous use people have them for. As people can only be in one usergroup any past use is gone. I have moderators who subscribe and moderators who don't. Using your system it would not work

If you do the extra profile field, the only problems are that it is harder to have a permission system for a forum for only subscribers.

As for setting up a subscritpion, writing a script that does

UPDATE user SET name_of_field = 1 WHERE userid=form_input is not hard, it is exactly what i do and takes me 5 seconds when i get a subscription notification.