I like that it integrates with slack which is very popular amongst our team members, as well we are able to share documents from cloud storage such google drive etc without having users download them first! It also has an option for internal mail but this didn't work out so great when trying to send emails internally within my own department (I have created multiple teams).
The only downside i can think of right now would be if you're looking at using something else than email/slack integration then maybe consider other options? We used outlook before switching over entirely into slacks communication channel system however there was still some areas where they could improve upon their product - mainly being more seamless between different channels e-mail / chat & voice call systems all rolled together seamlessly.