can't you just use an open source library for this? how many feature flags would you even need for this to be worth it?

Good question - teams should do what’s best for them based on their resources and scope of feature flagging needs. Though open source libraries exist, they're per language (Ruby, Python). We offer support for all major languages + iOS. As well, we have environment support and access control levels to use feature flags effectively throughout development. You can read more here: http://blog.launchdarkly.com/enterprise-requirements-for-man...

Any hope for Android support?

Yes, we are planning on adding Android this summer. Are you interested in being a beta user? Write us at support at launchdarkly.com

It's feature flags + AB testing with a nice interface for non-programmers to twiddle with and analyze the results.

It's basically the same market as Optimizely.

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Has anyone tried Launchdarkly for this?

While it looks pretty slick, I wouldn't personally pay a monthly fee for feature flagging. Our (pretty basic) implementation is just over 50 lines of code, and allows us to do all the targeting we need - either individual users, or particular groups of users we've tagged as having the feature. It just doesn't seem worth shelling out extra for that.