[kcorbitt]

At Google every significant feature in our codebase was behind a feature flag. Pro is that you can roll out features slowly and do a statistical analysis of their impact on user behavior in a well-controlled way. Also, you can turn off a broken feature really fast without having to wait for a new deploy (especially important with native mobile apps).

The big downside was that it significantly impacted developer velocity. You have to test that everything still works with/without your feature enabled. Also, sometimes supporting the old and new way of doing things simultaneously requires a nasty hack, and when you go back in to pull out the old code after a successful deploy you don't actually bother to rearchitect things the way you ought to.

For Google they were probably inevitable, but for a small to mid-sized company I would avoid them.