We used to have our own feature flag library built by our CTO as well! We (thredUP) recently switched to Split (http://www.split.io/) and it's been incredibly helpful for our team of 25 engineers. Slow controlled roll-outs, short circuit broken features without a deploy through their dashboard, and we actually use them for A/B tests as well since they have an impression data pipeline and A/B tests are so similar to feature flagging. Definitely one of the smartest "buys" we've made in our last 4-5 "buy vs. build" discussions.
