What does Slack do well? I've used Slack with clients, as well as in courses/programs. I think it's great communication tool for teams. It's very easy to have both larger and small groups set up, and allow everyone to receive the info they need. It's a good alternative to email, and better allows you to communicate emotion, too.
What does Slack not do well? I believe it's really only necessary for groups of probably six or more. Anything less than than doesn't seem to necessitate moving conversations out of email or a project management software, in my opinion. The only other reason I probably wouldn't use it regularly is that it's just something else to learn and keep up with.
What do you wish you had known about Slack before using? I'm glad they have the Slack bot to answer questions and give tutorials, as it's not always an intuitive program when it comes to notifying one person, or the entire group of people, in the channel. Just getting thrown into it takes some getting used to.
Have you used a competitor to Slack? If so, how does it compare? I haven't really used a competitor, but for managing tasks, etc, I prefer a project management system rather than something like this.