What does Slack do well? I love using Slack. It's a great resource for an instant messaging tool. You can create channels and group messaging. My team uses it a lot for getting quick answer. We use channels that act as an archive so that we can all collaborate together. Functionality wise it's great, but it's also a very modern tool. Unlike Skype it's more visually appealing and has some fun features, like emojis and status.
What does Slack not do well? Overall I'm completely happy with Slack. There's an app you can download on your phone so that you're always in touch. Plus you can download it to your computer so that the messaging automatically pops up as compared to using the web browser version.
What do you wish you had known about Slack before using? I wish I had know more about the threads and channels. I know there is a way to invite an outside organization to join your group in Slack but I'm not sure how to use that. If I did I think that would be a beneficial way to work with contractors outside of email for getting quick responses.
Have you used a competitor to Slack? If so, how does it compare? My company also uses Skype instant messenger. The two applications work very similarly, but I enjoy Slack more. It's more visually appealing, easier to use, and I like that there's an app you can download.