Yes, using their pseudo-transactional campaigns. I'm using it right now on multiple sites. Which makes me wonder why they decided to build on SES, they've already got the functionality there...
Pseudo-transactional campaigns are very "pseudo". MailChimp campaigns require static predefined content. If you actually wanted to vary the content from recipient to recipient, you need to find a way to stuff the dynamic content into merge fields as you send which can cause latency and concurrency issues.
Also, since pseudo-transactional campaigns uses our sending infrastructure tuned for large bulk mailing, the delivery times are somewhat longer (on the order of minutes) for pseudo-transactional campaigns, vs seconds for STS.