As a QA (and later team lead) for Opera desktop Linux browser development I ran Slackware as my main OS for at least two years and never had any major issues with it. I was able to run any app I needed for testing (if it wasn't part of Slack I could easily get it from SBo or build it myself). I also felt I had less annoyances than some of my colleagues running other distros.

Admittedly, I later switched my work machine (of my own free will) to a dual boot setup (VMs didn't cut it when we started to focus on hardware acceleration) of openSUSE and Ubuntu. I did this because we estimated that we had more users running GTK3 environments like Gnome 3 and Unity (despite the vocal complaints they are popular with the masses), so it made sense to use a similar setup to most of our user base.

My takeaway from this was that Slackware works well as an OS for getting stuff done in a work environment. In the special case (that I find myself in) where you are developing software for users on other distros, there is an argument to be made for running one of those distros, so that you encounter the same issues that they do. Otherwise, run Slack!

P.S. Currently I am assisting the Mac team and as such I run Mac at work. As we ramp up on bringing the new Blink based Opera to Linux I hope to get a chance to assist the Linux team again, though for similar reasons as before, I'll probably run on of the more widely known distros, despite personally favouring Slackware.

We have a couple and some more planned.