We've used LaunchDarkly (LD) for about a year now. We previously used our own Feature Flag library built by our CTO: https://www.npmjs.com/package/dipswitch. LD provided a lot more, and continues to release new features that would've been much more expensive for us to build vs. buy here.

We’ve been so pleased w/ LD that we agreed to do a case study with them if you want to take a gander: https://launchdarkly.com/casestudies/lanetix.html

Also, we implemented most of LD in a 24-hour quarterly hackathon we hold at my company: https://engineering.lanetix.com/2016-02-22/how-lanetix-does-...

Hope this helps!