I'm not a MailChimp customer, but...
I've always been a firm believer in supporting companies and brands based not on the mistakes they make, but on the way they rectify those mistakes. That in my opinion has always delivered that lasting impression which gets my brand loyalty & commitment going.
Well-known publisher, GigaOM, had a case of e-mail blues today as they sent out truckloads of the same newsletter to subscribers (or in some cases, non-subscribers). I got 40-odd copies of the same newsletter.
It turns out that MailChimp is their newsletter provider and the issues GigaOM were experiencing was a result of a deployment issue on MailChimp's side. It would be easy for MailChimp to hide in a situation like this; if they didn't put up their hand and claimed responsibility, then nobody - except them - would've known that they were at fault. Instead they post a very public apology, pulling no punches in acknowledging their mistake and also rectifying the underlying issue ASAP.
That in my opinion is the magic stuff that great companies & brands are made of: transparency, honesty & accountability. I'm not a MailChimp customer today, but if I were ever to switch away from CampaignMonitor, MailChimp's actions in this regard would've greatly influenced that move. Kudo's to MailChimp for impeccable customer service.