Well I still get a fair amount of spam through mailchimp - well over 50% of emails received from them are from spammers who have harvested or purchased an email address.
Having said that, mailchimp are probably one of the most proactive bulk mailers in terms of responding to abuse reports. It would just be nice if they could stop it BEFORE the spam is sent. Here's a suggestion: fine customers $1000/email if they email purchased/harvested email addresses.
They do better than that - if you get high spam complaints on your email (>2% approx. equals high), your account will be locked. Even if you've been a paying customer for months. If you go >1% they'll put you on double secret probation and you have to explain why it's actually totally legit for one of your campaigns to have such a rate and promise never to do what you did to cause that again to get it reinstated.
The second part is from personal experience. Although not a spammer, any list that you harvest from third-parties has an inherently high spam rate and MailChimp will catch it fast.
