It's super easy to use, all you need is an account with them (which can be set-up within just 5 minutes) & then start adding subscribers! Their service also allows me filter out spam at source so I get rid of that too - win/win for us both :) They do charge per month but their rates are pretty low compared to competitors like SpamGator or CatchYourSpammers which offer much better value in terms of price point. Not as many features / filters than some other services available such as OptinMonster etc., however they did send emails through our autoresponder system without any problems nor complaints coming back once we implemented it into our campaigns. So far no issues whatsover since integrating mailchimp.