I rarely write reviews, partly due to laziness and partly because of busyness. But after purchasing 1 stack to try the service, I am so delighted that I thought I would share the experience with everyone. Here the basis of my rating:
1. Value - I do not know a better price for email validation.
2. Ease - It was easy to upload a list of bulk emails. The output was easy to read and actionable.
3. Quality - This is one of the few engines that includes an extra accurate option. (It's a bit too early for me to comment whether this works or not. But assuming this is not just for the show, it may be valuable.) Also, for some of the emails, The Checker provides an alternative e-mail.

This may interest those who are using this data in conjunction with another AppSumo offer - FindThatLead. I performed a comparative study to assess the relationship between The Checker's score with FindThatLead. For example, I tested 9438 emails from FindThatLead. Only 14% is deliverable (rating from The Checker) is scored between 90-99 in FindThatLead. If I were to look at both "risky" and "deliverable" from the Checker, then 73% of the emails are between 80-99 from FindThatLead. So from a marketing perspective, 73% is not too bad.