NOTES
Things have changed in the world of Campaign Monitor and Salesforce integration, The Campaign Monitor for Saleforce app is now by Campaign Monitor. .
The marketing says
"Campaign Monitor for Salesforce is free for all customers of both Campaign Monitor and Salesforce. There are some premium features available for customers to upgrade to. Pricing for premium features is dependent on your active subscribers."
Well the free version is quite useless, and you then end up paying double - once for Campaign Monitor and once for the Premium Integration. See pricing and features. So for $50k subscribers you are paying US$100 for CM for SF and US$299/month to Campaign Monitor, or US$505 per Campaign. On Mailchimp that would be US$240 per month (or US$500/campaign).
Look the premium version is pretty good, but you are going to pay for it. It is better than Mailchimp because you can automatically subscribe and unsubcribe contacts and leads from lists. You can also choose which Activity data to sync into Salesforce and how many days activity - the more activity you sync the more data storage it takes up.
Oh, plus they say they provide no support for the app! But I hope that is just a hangover from the old app.
Notes
Only has a 14 day trial period so it can't be installed until almost ready to go live.