Yup, I evaluated Slack and HipChat about a year ago and went with HipChat mostly because of Slack's lame Windows user experience. (IMHO, the Slack client also had too many bells and whistles. I would've like a default "simple" mode that just is an IM client with group chat.)
The funny thing is, I'm pretty sure the HipChat client is (or at least was) mostly just a shim around a web browser with some notification stuff bolted on. This doesn't need to be a huge undertaking.
Maybe it's changed since I last looked, but they didn't have a client at all. You could download Chrome and then install an "app" through the Chrome store. Well, some of my users prefer Firefox... I don't know, it just wasn't a great onboarding experience.
In retrospect, though, I might've gone too far the other way -- hipchat is pretty bare bones.
Slack's lack of a native Windows app sunk it for our org - then the rest adopted Hipchat and I think it is too late for us. I would love to use Slack, alas.
