From the Store description:
We’ve been working hard and feel confident enough to open up our Windows Phone Beta to all the patient Windows Phone users who've been clamoring for it. But this is a beta — not the finished version, so it has some rough edges and quirky traits unbecoming of a proper Slack application. We humbly ask only one thing: please give us your honest feedback so that we can make your Windows Phone experience great. You can provide feedback just by tapping the feedback link (in Settings -> About) or sending an email to firstname.lastname@example.org
Just as a taster, here's the changlog for the current version over and above one from two weeks ago - the Slack development team have been getting in at least one update a month:
Release notes for Version 2015.923.0.0 (September 23rd, 2015)
- Better handling of connectivity changes and app resuming. We still have work to do so it's faster & more reliable, but this should be an improvement
- Properly notifies if authentication settings have changed and a re-login is required
- Badges and tiles clear more frequently
- File uploads - sorry for the delay, the upload was stuck at 99% for awhile
- Until full native profile editing is added, a link to the web version of the profile editor has been added to Settings
The Slack team has posted a list of known bugs here, so that URL is worth bookmarking. You can grab the client on any Windows Phone here in the Store. Of the things that haven't yet been implemented in the Beta, the big one is probably searching, i.e. searching all messages in a group for a particular text string, but it'll come in time.
In lieu of me having any Slack experience, here are some promo images showing the application in action:
Comments welcome if you've tried Slack on Windows Phone (or Windows 10 Mobile) in your own company or organisation.