I love MailChimp. It does so many things really well, the price is great ( especially if you don't have a lot of contacts, since it's free for less than 2,000 ). It would seem like the perfect solution for a small business to send a monthly newsletter to it's customers.
One challenge I recently discovered, however, is that MailChimp really wants you to log in and create each newsletter. While it's certainly a good idea to create a unique newsletter each time, this is not always something a small business will have the resources to manage.