I'll add that the only issues I came across were from a few users who didn't like the mobile client as well as Slack. I would chalk most of that up to them simply being used to Slack.
But I think Mattermost could still use a little bit more polish (as pretty much all software could) when it comes to the clients. I hope you guys don't lose focus on that. I would much rather have a progressively better user experience than one more widget integration that most won't use anyway.
Fantastic! Thanks for sharing back--comments like this are super motivating for everyone working on the project,