Mandrill is a transactional email service from the makers of Mailchimp (a popular marketing email service). If you're already using Mailchimp for your marketing emails, using Mandrill for transactions is by far the best choice.
Pro Webhook Reporting
Mandrill provides and Event API also or "webhooks" through which it registers events when they occur. So when an activity occurs, the application server is called along with the activity that occurred. Events include opens, clicks, bounces, deliveries and spam complaints.
Pro Highly customizable templates
Mandrill has made templates a core part of the product as they simplify the payload when compared to sending the full HTML each time and allow non-developers to design the template.
Mandrill uses the flexible content formatting of MailChimp templates. This paired with the API access to template management, gives the user a great deal of flexibility.
Pro Clean UI
The UI is very clean and good-looking overall. While the Analytics may seem basic and not very detailed, the dashboard interface is very clean which also helps with the set up.
The left side of the page has the navigation to different parts of the dashboard. The main view displays all the most important data and analytics that you may need (emails sent, deliverability, reputation etc...). While the navigation itself is pretty straightforward and self-explanatory.
Con Has a bad history
When they decided not to support a free tier anymore they've set up a withdrawal period of only 2 months for existing customer before it stopped working (even for people who bought credits to send more than that free tier). Meaning people using it had a month to switch to another solution or start paying $20/month or more.