What does Slack do well? Slack is a solid messaging application through and through. Easy to use, easy to onboard, and tons of fun features like reactions and URL unfurling to help level up the messaging experience. On top of all that, I really like the way that different organizations can spin up contained chatrooms, so if we're working with different agencies, we can have cross-org Slack workspaces to improve communication. The pinning messages function is great as well.
What does Slack not do well? Search functionality is my biggest struggle. I find it very difficult to search, especially if I'm looking for files. I'm not sure Slack is completely to blame - I think naming conventions of files and pinning/starring messages with files would improve that.
What do you wish you had known about Slack before using? That they had the ability to create cross-organization workspaces earlier! It's a major value add, and I wish we'd had it earlier to improve communication with our agencies.
Have you used a competitor to Slack? If so, how does it compare? Flowdock - it wasn't as feature rich, the UI wasn't pleasant, and it never really pleasantly surprised us. Switching to Slack, the user engagement spiked substantially across the company.