I'm just looking into it and wondered if anyone else has had any experience with it.
Kleinbl00 is right. I use it constantly to handle all communications between me and my team. Its better and faster than email and more conversational. Here's another comment on it. Here's some screenshots of how we work.
I'm also part of larger IRC style groups (mentioned in comment above). Only benefit is I always have slack open so I communicate there more ofter. I rarely have an IRC client open.
Switched to it at work recently. I like it. Well designed, slick interface, lots of features. It's apparently really easy to add external services to it. I work with a small team that's known each other for years and works in the same room, so our use is a little... unconventional. It's easy to set up trigger words that cause slackbot to respond, which we use almost exclusively to sass each other. Good times.
we use it at work. it's good for emulating direct conversation.