Slack was fine for our team of ~10, but frequently blocked by Chinese great firewall, and overall experience was slow. We moved to a free self-hosted Mattermost instance and been using it for a better part of a year. Conversations are fast, we own our data, and there is no looming 10000 messages limit all the time. Bleve search is working just fine for us as well. Win!

Mattermost is an amazing product with a few quirks:

* it is hard to set up any kind of integration with a TUI: `matterircd` is quite buggy and also eats the CPU like there is no tomorrow, `matterbridge` doesn't sync correctly, or at least we couldn't make it work, and `matterhorn` is a standalone product (which would be much nicer as an e.g. weechat module) trying too hard to look like a web UI;

* web UI is still being polished, e.g. if you connection is unstable, web UI websocket is going to behave badly, requiring frequent refreshes to not miss any messages, or it still doesn't scroll to bottom sometimes on new messages, this has been the case since v.4.x and we're now on v.7;

* what I think is different from Slack is that threaded responses are shown in the main channel as well, and if there are many threads it is the same mess as if there were no threads at all :D

* somewhere around v.6 update the Focalboard push was a bit jarring, thankfully after disabling the plugin it disappeared for good.

Keep up the good work! Once our org is larger and requires Enterprise features, I'm pretty sure we stick with Mattermost.