I am using it for some clients.
That helps to have things managed very nicely. Conversation at one place for specific topic also a big thumbsup !

In my opinion because of its easy usability. Slack is way better - easy and clean than Skype(personal view). I was comfortable with it from very first day but still juggling in Skype between passwords,invitations ,for business and for non-business.? 

When it comes to maintaining a professional communication, both the ease and the environment of the messaging platform is important. One wouldn't use a platform like Slack for having day to day communication with their friends. They usually prefer Whatsapp, Hike, Skype, or even other platforms but not Slack. 

Hence, it easily separates the professional discussions aside and avoids the trouble of sending some non-professional messages into a professional group. Further, Slack allows creating Workspace which I think is great as it helps in managing both individual and group communication at the same time. 

Big fan of Slack. I've used Lync and Skype in the past, but have had to struggle every time I had to refer to an earlier discussion/attachment. Slack makes it so easy to revisit older discussions, and I like how it lets companies organize teams via #channels - all unified and accessible with a single click. Say If I'm working in two teams/projects at the same time, I can communicate with both teams using a single chat window. Huge win.

And that it lets you connect your Google Drive files directly is another plus.

Why slack? Because it allows me to have a conversation with myself. Most of the time, I find it way too mundane to use any note app. It just doesn't give me a the +ve vibe I require. With Slack, I can just note down the important stuff and paste it into a message to me. 

Except for this one, I like its UI. It works as a chatting application but with more features that I can add or delete features as per requirement. It treats a group or company as a whole different world itself. It allows you to have a reminder, to set up calls, have a 1-to-1 conversation or 1-to-many conversations via a channel. So, as in whole, I like the Slack environment.

you have chat threads in slack,

Sometimes when customer give me list of bugs then I can directly create a chat thread for each issue individually,

2nd Slack have it's own set of API, I can sync slack chat with third party php application

Slack has an API. 

I automate various things (trigger based communications) to my team and life is good :) ... Skype or Hangouts or WhatsApp doesn't give me that flexibility that Slack does.

Some examples of how we @ 13 Llama Interactive use Slack - 

1. Message #maintainenance channel when the site goes down :)
2. Every day post an update on the #adwords channel of how our campaigns are doing 
3. Every monday post updates on the improvements for our campaigns
4. We also use Trello, so when cards are updated, message respective channels for updates
5. Linked our website's site chat to Slack. Team members on Slack can chat to site visitors
6. When a lead is sent and its an urgent lead, take for example any lead that goes from our PPC offerings page, gets sent to slack to the respective campaign owners (me)
7. When the brand gets mentioned on Social Media, an IFTTT recipe posts that on the #social channel

Need I go on?

I've experimented with sending alert messages directly to Slack and it works great. 

I'm thinking of creating a mini-clone here for our CEans to interact. It'd be cool, I guess. 

I have not used slack so much , but I use flowdock a lot and I see it has all the facilities that slack has.. so why do we use it- easy to create /communicate/share videos/ images and tables, and it works fast and easy!!

my director used to keep this in his status flowdock>>> phone/email :)