Similar story. I really liked Slack and the integrations are fantastic, but the UI has way too much going on and the lack of a native Windos client meant even just deploying the app is a hassle. The mobile client has menus that slide from left, right, and top last I checked. I told them they'd have had my business easily if they had a one click windows install and an optional Dummy Mode I could turn on for users by default that makes it work like a simple irc client.

I went to HipChat instead but it's got its own problems. Weirdly you can choose to get notifications for all rooms you're in or none of them, but nothing in between. Very frustrating.