I've recently completed an email newsletter for a client, which would have been a difficult and time consuming job if I had to set up everything manually. However it turned out to be a streamlined, enjoyable and educational project because I used Campaign Monitor, which handles every step of the process, and handles it well.
Unlike other email marketing software I have tried, Campaign Monitor has a clean, usable interface, intuitively handles subscriber management, allows clients to obtain reports on their campaigns and doesn't mangle my standards compliant XHTML. It also responsibly handles bounces and advises on how to avoid sending an email campaign that may be mistaken as spam.
One feature request I would like to see (and I have asked for) is to allow an option for clients to manage their subscriber lists from their client login, if they client wants to. At present, they need to use my admin login to do this. I am hesitant to give my admin login to clients, as they then potentially have access to my billing information, as well as other client's email campaigns.