This is interesting. How many employees are using it? Slack costs 4 times as much as HipChat ($8/user/month vs. $2/user/month) so it seems silly to jump to a much more expensive product for no reason.
Developer tools generally charge far too little for their offerings - price point is probably not even a consideration for most companies, as long as its within an acceptable band.
But does it [compared to HipChat]? (asking as someone who has used HipChat but never Slack)
I don't think it's meaningfully different or better than HipChat. The most obvious differences are that it automatically retries sending messages if your network glitches, and that it's proponents are more irrationally positive than HipChat's.
They're interchangeable. People who argue otherwise don't have enough actual work to do.
If someone at the company wants it, price point shouldn't be what's keeping the company from using it.