I like how easy it was for me to set up rules using their free trial version.
It's very intuitive! Their customer service team were super helpful too when there were any issues or questions about my account setup (I had some initial problems but they helped figure those things out quickly). They don't have an API at present - which could be seen negatively if you're looking into integrating something else along side them. We needed better filtering options than what Gmail offers us so that people can actually find relevant content via search rather than having to read through every message one after another.