There are too many thoughts about Slack. Some of them are positive , some of them are too way negative 
At Onedio.com we are a team of hundreds of people. And we are all communicating with Slack. Some of our colleagues use only Slack to do their jobs. We have many webhooks and custom channels, like #daily-reporting-of-somethin, #comment-moderation-channel or #server-status-reports-channel.
We are communicating only with slack. Some of our friends haven't even opened their email inbox.
We have some private channels and we are talking about almost anything in these private channels (side projects, fashion, football, politics, gags, gifs, food, party, gossip, news etc). We have also too many public channels that are open to all users. We like open communication in our company.
Our slack usage increases by time.
Slack is super fast while transferring files.
Slack is just a tool. But it's a great tool. And it is working. Again but you will use this tool, and you may use it wrong. Maybe It's all about your communication problem. If your boss sucks at communication, sorry but Slack won't help your communication.
We are using slack as a natural way of communication. Our users are not always online. And sometimes they set their status don't disturb mode. In that case we are using different types of communication methods just like SMS or phone call (Btw slack also has an audio call feature). This is just a simple flow. If you don't like realtime notifications of everything then please use slack channel settings to turn on/off. If you like weekly/daily reports of messages then again use your slack preferences and make your settings with this way. Slack will help you to find your way on natural communication.
Slack works. Slack has a great dev team and all applications are just works without bugs (yes maybe with not very often crashes or UX fails)
Slack has many plugins. You can also write your own service or webhook for slack easily.
I am a fan of IRC and mail lists. But this is different. If your team is very young, dynamic and your project is fast growing, you will need a fast, clean and working method to communicate. Slack is a very advanced version of IRC.
1. Wht Slack Works? https://medium.com/@noelsequeira/slacktivism-cb384450e99e#.k5l2tsagb
2. Slack, I’m Breaking Up with You : https://medium.com/better-people/slack-i-m-breaking-up-with-you-54600ace03ea#.nd5hb27ci