My impression - the people who use it historically (and I've met a few) aren't the companies that typically proponent open source software and the like. They are "closed source" shops (e.g. finance, insurance, etc). Even I am not willing to post on more than a throwaway account. I'm currently using it for large scale production systems in a very large public company powering a large data volume product with very high peaks of customer traffic and it works a treat. We decided to try C# 9 after some F# because "higher management" - compared to F# the dev's have found it verbose and painful still, despite the new features.
The productivity benefits are small and sprinkled - I don't personally think there isn't one "killer feature/app". It isn't just one thing, its little things that add up. Given the team jumped from other ecosystems (e.g. Go/Node/etc) they found F# easier to approach than C#. This is the perspective of the team I run and it comes up in PR comments (I do less code writing these days). Comments like "don't have to do this in F#", or "we need a framework for this because C#" are known to occur. Easier unit test writing, less dep injection headaches, concise function passing, easier inlining of math for perf, easy mocking/stubbing, unions, etc etc.
The big weakness to me is that the people that use it typically don't want to flaunt it, and that means good mentoring, the best/simple patterns to use, etc and management buy-in are not really public. Communities that you can join are not into large scale apps, meaning good scalable patterns and lessons learnt are hard to find.
The productivity benefits are small and sprinkled - I don't personally think there isn't one "killer feature/app". It isn't just one thing, its little things that add up. Given the team jumped from other ecosystems (e.g. Go/Node/etc) they found F# easier to approach than C#. This is the perspective of the team I run and it comes up in PR comments (I do less code writing these days). Comments like "don't have to do this in F#", or "we need a framework for this because C#" are known to occur. Easier unit test writing, less dep injection headaches, concise function passing, easier inlining of math for perf, easy mocking/stubbing, unions, etc etc.
The big weakness to me is that the people that use it typically don't want to flaunt it, and that means good mentoring, the best/simple patterns to use, etc and management buy-in are not really public. Communities that you can join are not into large scale apps, meaning good scalable patterns and lessons learnt are hard to find.