We started using Mandrill about 18 months ago and it worked just fine for us. We used it because we were bootstrapping a company and had a need for email but it was very minimal. We needed a good free service and Mandrill fit the bill... Then they switched their policy and eliminated the free tier. Bottom line - it's a really good service as long as you understand what you're paying for up front. We do understand the need to make money and but the transition from freemium to paid was not graceful. When we can spare the engineering effort we plan on moving away from Mandrill.
If you are looking for simple email transaction integration with your app, Mandrill is one of the best. Though it's not longer available as an independent subscription, take advantage of Mailchimp for email marketing too. I wish the email log can be unlimited though.
Time used: Less than 6 months