It was a best seller before Juniper started using JunOS on their new firewalls (SRX) and it continued for a while due to being rock solid and their ability to do pretty much anything in a fairly easy way.

I remember working with my first SRX in 2009 and nowadays I still talk to customers rocking ScreenOS devices. I've lost any trust on them due to this news, of course, but boy their OS and feature set was awesome... I did like them so much that I bought a tiny NetScreen 5GT a few months ago for £10 only to gather dust on my desk for weeks, until I gave it away to a colleague that had a better use for it :)

Anyway, I derailed. They were everywhere and they're probably still around in many SMBs.

I got 8 boxes deployed for ~1,000,000 users