I have a few problems with it as a concept. It's web based, which is fine and everything since at least it doesn't require another goddamned client, but it also makes every conversation available by virtue of being searchable. Which just means that some conversations will wind up "out of band" because somebody doesn't want to communicate on the record (I know there are direct messages, but they're analogous to email at that point). Which just pushes everything off onto another goddamned platform. AGAIN. And it's one more thing to back up. And it's one more thing to administer. And why the hell not just use Email, which already does the things that Slack does?
You can use Slack with the webclient, but there's also clients for OSX, Windows, Linux, Android, iOS, and even Windows Phone.
Slack is a hosted solution, so you don't have to worry about backup, administration, etc.
Email is a terrible solution for group chat or even direct communication. With Slack and tools like it I can see everything ever sent in a channel without having to search Outlook or Gmail. It's much easier to share files, code, etc. When you're working on a big distributed team, email just doesn't work.