We have been using HipChat for the last two years for one of our two major products at work and have very recently moved to Slack for the other. While notifications on HipChat work most of the time, they are still very patchy. I personally use the Mac client, while others in my company use the web, Windows, and Linux clients and we've all been bitten by missed notifications on applications one too many times to actually become a nuisance for us. Until the very last update to the Mac client, I became used to frequently finding the client freezing up on me. The windows clients, on the other hand, is even more infamous for this. Plus, the iOS app isn't something I really look forward to using (judging by the fact that I've had it installed on my iPhone for far longer than the Slack app, yet I leave it signed out almost completely, while I enjoy using the Slack app).