It's great for small to midsized teams. I use it for all my projects and it's superior in a number of ways. I'm also part of two IRC-style chats which I love. (http://fedsonslack.com/ & http://freelance.chat/)

1. Everyone on the team stays passively aware of other things that are happening. Example: We updated the way we were handling plugins by default which, of course, led to some issues when starting new projects. However, since I knew that Team Member 1 had encountered the same problem last week, I was able to search, read the thread, and solve the problem. I was not involved in the project where he encountered the problem so I would have never been included in the emails.

2. During the design process, developers often chime in uninvited about certain things or issues or for clarification. This allows the writers and designers and developers and project managers to actually learn what other people do, and leads to better teamwork on projects. This is especially helpful after three rounds of revisions with the client when the people who have been staring at and working on the design for over a month are somewhat over it. For example: Last week, I had UX go through and add error message designs for a page that really needed an extra bit of thought. It is doubtful that I would have asked for a design if I had noticed this a month into development and we would have just used standard whatevers.

Another side effect of this is people on different teams are more likely to talk, discuss, and ask for things. I would rarely, if ever, email the API guy to ask a question about some hook or whatever. But in Slack it's informal and easy enough to do that which makes my life easier...and his. The more informal environment also makes feedback or "hey -- you should do it this way" happen way more than I have ever seen it happen in emails.

3. During final sprints or post QA revisions, we work together instead of on top of each other. You won't email your team saying "Hey - working on the team member page now." In Slack, you might, or your time-tracking app does automatically.

4. Search, save, file upload, and integrations into trello, dropbox, harvest, github etc.

5. It tells you when Hubski is down.

6. Emojis. We use emojis a lot to clarify tone.

7 & 8 & 9 & 10. CUSTOM FUCKING EMOJIS.