I work for an advertising agency and one of our clients uses Exact Target for all of their email marketing. The client has numerous business lines and each of them has their own email marketing needs. We average about 10 emails per month. Some of the emails are automated and some use data extensions.
I personally only have other experience using MailChimp which is great for smaller clients. ExactTarget is a lot more feature-rich and makes it easier to customize emails with data extensions. MailChimp is great for out-of-the-box, template-based solutions, whereas ExactTarget requires a lot more setup time. The two platforms have a totally different user-base and provide entirely different services.
ExactTarget is an extremely feature-rich platform and is only cost-effective for large organizations. To use it for email marketing, be prepared to know how to design and hand-code HTML emails. There aren't pre-made templates to use.