I've probably used half of the mailing systems out there including those for Joomla and others that are web-based (MailChimp, aWeber, etc.). I have been with Acajoom for the past two years and thought it couldn't be beat.
I was wrong. AcyMailing is the best, hands down.
This is one great module. It works great. The interface is clean and even stepping up to more potent versions is very reasonable from a pricing standpoint.