I too signed up to BT Anytime on Friday after becoming fed up with the quality of the service on ISDN Free Off Peak over the recent month or so.
I shall also benefit from the reduced rate for a couple of months until 1 Apr.
So far its good. Same number, 0800 731 7963 works for ISDN or V90 and it doesn't seem to matter which number I dial out from as long as I log on with the correct User Name and Password. I don't see the need to use their dialler package if you make your own DUN phonebook entry. I don't use CLI block (141) so I don't need to unblock it. Can't see it would matter if I did block it.
Its going to take a while for it to sink in; it still feels peculiar trying not to think about the 'phone charges!
So I would recommend BT Internet if you want a clean service with the least intrusion of the three ISPs now offering Anytime. The other two are FreeServe and AOL.