Love it or hate it, it looks like Slack is here to stay.
So, Slack just bought HipChat, perhaps its largest competitor, and the only option enterprises use since it has an "on premise" version. I take it that this effectively means that even the people in larger companies will now get to reap the benefits of a workplace with "less email".
The thing with Slack is that it's perfect for people who's whole job is to communicate, and a nuisance, another channel of interruption to everyone else. I honestly don't know a single "maker type" person that has anything positive to say about Slack and how it affects productivity.
I personally am of the opinion that we don't need tools that will make everyone more available, we need discipline and responsibility within our existing correspondence mediums — cough email cough — which are perfectly fine if you ask me.