I have been using MailChimp and I found it the best choice for Newsletter and building email templates.

The only annoying part as a developer is when I started using their API through mandrill, The API itself work perfect, but the issue is in the templates.

I start by creating a template in Mailchimp then submit it to mandrill (this gives me the impression that API is a second citizen in mailchimp, but not a big deal, it works) then I update the template code through Mandrill and here start the problem because you loose the Sync between MailChimp and Mandrill. Mandrill has a very poor template editor and you can't Sync it back to MailChimp to use their greate template editor.

Everything else is great.