What does the software or service do well? Kill email AND meetings with one tool!! There is certainly a learning curve for people who never used IRC or any kind of chat app, but once people get used to it, Slack is absolutely the best way to communicate and collaborate short of being physically together or over video conferencing all day long.
What does the software or service not do well? Trying to find old conversations, attachments and links from random conversations you've had can be a difficult task. However, Slack really shouldn't be used as a place to organize files etc. Another shared storage tool like Google Drive, Box, or Dropbox should do this. Even though Slack has functionality here, it's shouldn't be used that way as it's pretty clunky. They have improved their search functionality, but you still need to semi be a DBA to easily find what you need in the annuls of chat histories.
What do you wish you had known about the software or service before using? We wish we would have known just how expensive it was going to be, but I still have to say it's been worth it. We started off with only our IT department using it, and the costs were manageable. Now our entire organization has been adopting it, and it's been kind of a chore figuring out how to get this much larger budget item covered across multiple domains with separate budgets. That's obviously a problem that's probably limited to us though.
Have you used a competitor to the software or service? If so, how does it compare? We used to use Skype for Business, and we can definitely say this is way better and faster for everyone.