Can't understand the poor reviews below, to be frank (v 5.1.6).
I find MailTags invaluable. Mind you, I have been using it only since 2005. So my experience is perhaps more limited than for some. And it's limited to those Mac OSX operating system versions effect at the time ... the latest being macOS 10.13.3.
Users should keep in mind, Apple (in the past at least) has not provided developers with a good, clear way to enhance Mail's functionality. Therefore, all Mail plug-ins will be less perfect than otherwise would be the case. I use several, including MailTags, Act-On, Daylite, etc.
That said, I could not use Apple Mail without MailTags above all having to catalogue and track tens of thousands of messages. And I have had no stability issues except when using beta versions of the software -- where instability is part of the game.
So while some folks have had a poor experience, I have not.
BTW, the developer has always been responsive to my queries.