The best feature is that it allows me to send my clients timely reminders, which are essential in this business of mine where I am constantly reminding people about upcoming events or deadlines! Sometimes though when you get multiple messages from someone they can be overwhelming but overall great product so far - nothing much as yet (although considering their price point its hard not too!).
Definitely worth looking into if your wanting something with an interface similar to Slack/Hipchat etc.. Great software at such reasonable prices also makes life easier by allowing us all access through our phones rather than having everyone come together physically each week :).