We started off with AWS SES, but found it to be lacking in diagnostics and reporting. Mandrill is a quantum leap forward in this regard. Has some other really neat features carried over from MailChimp (templating).
We use Mandrill to send transactional emails and newsletters. We've just moved from SendGrid as Mandrill has far better support for templates and also integrates really nicely with MailChimp where we can design the emails.