We regularly use MailChimp and Highrise together to create new mailing lists and track campaigns. Here's an example:
1) In Highrise, I assign a tag to a specific group of people I want to contact, perhaps just "Clients".
2) In MailChimp, I create a new list and tell it use contacts from Highrise with the "Clients" tag.
3) After the campaign is created and sent, MailChimp will then create a note for each of those contacts within Highrise describing what time the email was sent to them and how many times they've opened or clicked on items. MailChimp also creates a new case in Highrise where it lists all the details for the campaign.
I think MailChimp provides updates for the campaign for only 2 days in Highrise, but it's still very easy to set up and gives enough information for what we need.
A sample MailChimp newsletter.