I haven't used Slack, but I've used all sorts of xmpp/jabber servers such as Openfire, generic jabberd, etc. Hipchat blows those away. I can quickly see what links and files I've posted or have been posted to me. I can open unlimited rooms, invite members to them and if I have a new employee start all I need to do is make them an account and they have everyones contact information. We previously used g-chat which was an absolute pain. Half of the employees I have on gchat I have their personal emails.

And it was about $3/mo per user. Not bad at all.