I've been working with engineers writing production Haskell for the first time; its lack of a "null" concept is proving to be absolutely incredible. In a high-uptime production environment, it's at least as valuable as Haskell's enshrinement of purity.

We have a convention that all recoverable errors be captured in Either or Maybe. This policy is paying off in a huge way because the type system forces us to think about what error cases mean. This is driving us to write substantially higher-quality code than I've written with other tools.