What does Slack do well? Slack makes communication so much easier for our remote team who is constantly traveling to clients all over the country. It cut down our internal emails and allows us to discuss each client in a separate channel--much more organized and seamless.
What does Slack not do well? I wish Slack had a better way to on-board non-tech-natives. The tool works great if you use it, but some people are reluctant to use it.
What do you wish you had known about Slack before using? That you had to pay to keep historical data. I'm surer Slack mentioned that many times, but you never think you are going to use that much.
Have you used a competitor to Slack? If so, how does it compare? Yes, I used Yammer and Goggle Chat at my last company and they were bulky and not fun to use.