I am a big fan of Slack since the day 1. I have been using Slack from past 3 years and there's never a dull moment with it. With Slack app on the phone it is like I am conveniently carrying my entire workspace with me where I go, making my life so much easy.
I can stay in touch with my team, check my tasks, update my tasks, discuss on them or even collaborate with my google docs and finish small tasks like editing all from one app and that too while I am on the go.
Also, i love using other features like channels, threads and espcially its reminder feature. I simply set a reminder and the Slackbot will remind me or anybody from my team whom I have tagged to do a particular thing/task on a stipulated date and time. Isn't that awesome!
You can upload documents, pictures and videos as well on it, making it easier for the team to communicate with different media files as well.
So I guess it is certainly a great messaging app to use for software development companies whether big or small.