I used MailChimp for 6 months, but it's pricey if you've got tens of thousands of addresses.
I use SendGrid for transactional emails, and a few months ago they added marketing email features - and it's FAR less expensive. Yes, they haven't got all the features figured out yet, but for the cost savings, it was worth it to me.
MailChimp lets you put your account on 'hold', which I did, and I've now sent several campaigns through SendGrid.
For MailChimp, I was paying $300+/mnth to store and 'manage' the email addresses.
For SendGrid, I'm paying $10 per 10,000 email addresses, and when I send campaigns, each message sent comes out of my monthly plan.
Overall, I'm saving several hundred dollars a month, with limited loss of features.