I'm trying out Slack. Here are some first impressions.
The system has many familiar components. It feels like an IRC network, complete with a set of well thought out bots and useful integrations with development tools. Unlike most IRC nets, it comes with a rich set of web and native clients that make it feel like something that ordinary people could use and not just the ubergeeks. For the ubergeeks, it also has a very competent IRC client support.
The experience of the system varies tremendously depending on who you have on your team. If they have somewhere else to talk, then you might never get the critical mass you need to have a real conversation online. About a third of the people I'm inviting actually start to use the system, and most of those have some IRC channel time in their background.
The web client and the Android client for Slack are both very capably executed and feel like sound, solid pieces of engineering. It adds a lot of confidence to using the system to have such nice tools to work with.
I have integrations configured for RSS feeds, for Github (issues and commits), and for Google Hangouts. There's a rich API, or so it seems, and lots of other systems that have had integrations built for them. I have ambitions but no infrastructure yet to write my own code or better yet borrow other people's code to extend the system. I'm not much more advanced yet than getting the "hello world" style post-to-Slack-from-the-command-line capability going, but it did work, so I have high hopes.
The slack server I have set up is running on their free trial, which has a limited number of integration slots. I think that by cleverly enabling and disabling services that I can try enough of them to build up some experience with the system, but not necessarily run everything in production. I get 5 for free, and Github, Hangouts, RSS, Twitter, and the incoming webhook make up that 5.
If you'd like to see what I've built and join in, drop me a line and I can send you an invite.