I am not a fan, I think it has reshaped our communication to our detriment -- repackaging our availability as a feature.
What has been your experience? I'm particularly intereted in hearing from people who value it.
There's nowhere to hide with Slack. If you say something, you have to commit to it. Too often in the past I've been working on a project and someone may commit to something on a call, but end up changing tune later on. If we didn't send out actions/decisions and get agreement from them - these decisions or opinions are never logged.
Also the previous "this could have been an email" meeting responses we used to have has been massively curtailed because we're all happy to chat in a Slack room. Slack has made chat usable by more than just engineers.
I feel bad for the Open Source community, however I believe there's plenty of tooling available to rip Slack data out of the platform and put it into something else. It just needs to be easier and more commonly used by admins. Similar to what we see on those IRC history websites.
Before Slack I've used XMPP, and Slack is much better because it supports mobile, does not lose messages when you go offline, and has archives with functioning search (yes, it is not great, but at least it exists...)
Cannot say anything about open source, but for commercial it is sure better than alternatives.
As for open source projects I might rely on, having a slack but no active mailing-list/forum/async-channel is a sure red flag.