I am using it as an SMTP for sending mails from WordPress. It works flawlessly, robust insights, a lot of data, and very straightforward setup. A really generous free plan coupled with a lot of really cool features. It also provides marketing related tools as well, including ability to create campaigns. I personally use it for transactional mails only, but it is a really great tool for a majority of use cases. Review collected by and hosted on G2.com.
Not much to dislike about it. However, the inability to view an already sent email is a bummer for me. As someone who sends 1000+ monthly emails, I don't always remember what I wrote in a particular mail. There should be proper capability to view any sent transactional mail. Review collected by and hosted on G2.com.