It's dead simple. Pricing is great. Their API is the easiest I've used although SendGrid is a close second. Their interface is likewise straightforward both for sending and also handling incoming email. It's the perfect solution if you're already using MailChimp their parent service.
It's now only available as part of a MailChimp subscription. This is both a good and bad thing, good if you already use MailChimp but otherwise this service is no longer available as a separate solution.
If the product or service you're building already uses MailChimp then Mandrill is perfect. But otherwise it's no longer even available as they have shuttered their standalone service.
I've used Mandrill for 3-4 projects. For incoming email they also let you couple specific email addresses with webhooks. Other providers require a more complicated setup with subdomains rather than email addresses. For starting a new project they generously allow you to start with a free plan up until 12,000 emails a month. Email tracking (opens/clicks) for transactional emails is also one of the best.
It was straight-forward to link to my WordPress site and it's easy to monitor emails that are being sent from my site.
Support is very limited and slow to respond and they've had some recent technical issues.
I recommend looking elsewhere.
I wanted a reliable way to send and monitor sending of emails that are sent from my WordPress-based site. For the most part it has worked reliably, but recently I've begun to question the future of this service.