We have a customer using Sendy for about 6 months now, no problems, about 100k+ newsletters sent.
Agree with @Sergio, keep an eye on SES dashboard as it's feedback is not visible in Sendy.
We integrated it with Processwire, in as much as we built simple pages(form) where end user adds images and text for a newsletter. This then gets sent to Sendy as HTML Email and a Campaign all ready to go.
The concept was to simplify and shield user from complexities of making HTML EMails. We used the Zurb Foundation as the base templates for the HTML EMail code. With Processwire we then merge template and end user form input and send to Sendy via it's API.
The calculator on the Sendy.co web site is correct. The AWS SES cost to date for the 100k+ mails is about $USD 10. The clients savings from previous provider for same qty of mailings is approx. $USD 1,000.
For a small business this really is significant. Actually, to any business.
However, companies like Mailchimp and others are providing a good service, pre built templates, email designs, wizards, security etc... which you are going to have to do yourself to some extent if you go the Sendy route.