The basic free version is exceptional. Very simple to use and well documented. I can mass-subscribe users myself, get them to create / log into their Community Builder account and develop a relationship with them afterwards. Extremely intuitive and highly recommended.
If you're running this professionally for clients, I would upgrade to the full version for high-level statistical analysis, cron jobs, campaign functionality and more.
The one word of caution I would say is that styling HTML emails for multiple browsers, webmails and Outlook 2007 (grr!) remains a horrible job whatever you use - don't blame this fantastic bit of software for that - you still need to know what you're doing!