I like Slack and have been using it since very early on, but although it is pretty my use of it has tailed off and I've returned to sharing things via email. It essentially works like a private Facebook wall or any number of other shared-space aradigms. It's beautifully designed, and it just works, but once you fill it up with a few months worth of random crap it's not obvious that it's fundamentally better than any other social communication tool.