Sure, it's replaced our documentation wiki altogether. People really like it and it's full of useful information. I think it certainly is worth the cost. We have a board for newbies with a lot of "cards" with useful information. Things like, how to start building a service, how to setup IDE, how to connect to different places, how our CI works and so on.
That's true, many of the conversations where devs are looking for a specific piece of knowledge originate in Slack. There are also other tools such as Obie, etc also integrate with Github to help close the loop.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.