Mark and Nikky: RMail seems to be the best for two reasons: Firstly because it seems on average to send the notification emails more promptly, and secondly because the format looks better and it includes the title of the latest post as the email subject.
http://markandnikky.blogspot.com/2005/08/how-do-email-updates-work.html
Randy: I need an Rmail blog to capture all this great feedback.