Me and my team are planning to move to mattermost, but their Slack import just isn't 100% yet, so we've been holding off on it. For example right now it doesn't import the chat logs of any bot account.
I'm concerned about the mobile-friendly options for MM. Slack has pretty good iOS apps (and others, but I've never tried them). Does MM have quality mobile apps?
On my android phone, the mattermost app does something weird to my keyboard - it replaces the right-most 1/3 of the spacebar with a key with a '->|' symbol on it that closes the keyboard when pressed. I've never seen any other app do this, and it's (of course) really annoying because it means when I try to hit the spacebar the keyboard closes instead.
That's just one of a couple issues I have with the app (push notifications not really working, the chat taking 15 seconds to reload everything upon switching back to the app, etc) that have basically made it useless to me.
https://mattermost.atlassian.net/browse/PLT-48
While we're not in the biggest rush to move to Mattermost, it does seem to offer more than Slack right now.