My current employer uses hipchat, and I've used Slack at previous jobs, social/professional groups, and with some of our contractors now.

Features: Slack wins. Slack is usually a little ahead. Hipchat was catching up and then I just popped back with threaded convos.

Uptime: Slack wins I think. Hipchat goes down/slow kind of often. Approximately github often. I don't use Slack as intensely, but can't think of many times that slack was down aside from major internet outages.

Rooms/Channels: Hipchat has rooms, slack has channels. Basically parity in terms of group/private/team channels.

Pricing: Hipchat wins. Both have free tiers, but those are for toy groups. Hipchat is about $2/user vs $10/user for slack. Slack doesn't charge you for inactive users though (2 weeks dormant), so it can be competitive if you have a small team of actually active users.

Mobile: I find hipchat to be slower and a little worse in tiny annoying ways. For instance on my phone/watch, I get notifications for 1:1 messages and @mentions. But in hipchat, if i'm not already in the room where someone mentioned me, it doesn't show up when I open the app, even if I get there by swiping the notification. Come on hipchat!

Feels old, clunky, and not fun to use compared to Slack. The hosted solution has also had major reliability problems over the past 6-12 months (may not apply if you use the on-prem offering). The mobile apps are very outdated and tend to botch notifications and message delivery/syncing.

On the bright side, the desktop client uses very little RAM compared to Slack.

I use both daily. Honestly, I don't get the fuss about Slack. It's the exact same service.

The differences in Slack are emoji attached to messages and threaded replies that just came out.

The hosted version doesn't have support for kvm, and until recently video chat required an internet connection.

it's been around a while, i'm not sure why it never took off the way slack did.

i'd also love to hear from a user.