I really like Slack but I think it will be troubled in the short/long term in the same way Dropbox is now. It is innovative but many others will catch up it soon.
I look at Slack as being more like a hybrid of the best parts of Box + Dropbox... They have the good parts of Box's business model and the UX quality of Dropbox. And fortunately for them, they aren't in a market where the price is driven largely by the cost of the commodity hardware underneath them.