I've been using it since the beginning of this year, real glad I gave it a shot. If there is one thing I hate debugging, it's email.
Care to elaborate? What does sailthru do better than Mandrill?
It does you no good if you use premade apps, but API based services like sailthru and postmark are infinitely better if you're a developer.
Reason being simple, smtp is a costly protocol time wise, and the operations are blocking. So to say, if you need to mail 3 clients at a time, you need to open a connection, authenticate, send, close and repeat three times with most implementations.
To transform this into non-blocking ('queuing') based operations, you either have to use something like iron.io, or Amazon SQS, or Beanstalkd, or completely ditch SMTP and go with something new.
Now, HTTP api based services like the one hazardous mentions work somewhat like this,
You call API::send($this->mailDetails), check for exceptions thrown, and call it a day. The service handles delivery,
AND queue processing for you, and all in the same time, the operation stays non-blocking so you can proceed to the next breakpoint in code.