It doesn't have the slick, "just works" appeal of MailChimp, but it also doesn't carry the big monthly price tag.
We've had the same open and clickthrough rates with Sendy as before, and I haven't encountered any problems getting marked as spam. That said, our entire list is people who actively came to the site and signed up for updates.
If price were no object, we'd go back to MailChimp in a heartbeat. But they were charging us $125/month just to keep our mailing list on the service, whether we sent anything that month or not. Just not a good use of funds for us at this stage. Sendy is great at our current size.