As far as usability and all that I really like Hipchat. I haven't gotten the chance to use Slack, but I've got no complaints on the Hipchat application itself.

As far as usability and all that I really like Hipchat. I haven't gotten the chance to use Slack, but I've got no complaints on the Hipchat application itself. My current megacorp employer uses it as part of a big contract with Atlassian, but ours is hosted internally. I don't know the intimate details of self-hosting, but I assume that mitigates most of the security concerns.