In the early stages of developing something complex you often want to play fast and loose with the type system - get some code running, and some tests set up, and go from there. But later on you want to refine it and start catching some more errors statically.
The pendulum has swung really far to static typing - and I get why. But if it swings back I hope we don't forget about gradual typing.
The poster child of gradual typing has to be TypeScript, and it's been a wild success. TypeScript allows gradual implementation to any JavaScript project and there's almost a million small different gradients you can use. From simply using a js file, to noImplicitAny, to simple types, to using complicated built-in types (eg Partials).
With TypeScript, you never lose the speed of dynamic typing. The whole point of gradual typing is you can jump between more and less types, so at no point are you restricted from reverting to full dynamic typing. And it is true, you can't rely on all types being checked, but the point is you add it to the important/stable bits. And having your largely stable important bits being typed while your more experimental unstable code can be dynamic (while still getting the benefit of consuming the stable types) is a nice place to be.
As I said, it is what you make of it. If you spend all your time typing the unstable bits and leave the stable core bits untyped, well, you've just wasted your time and would get the worst of both worlds.
I believe Haskell has them, is that where you've used them?
I mainly use Swift these days but I haven't had a need for them yet so was asking out of curiosity.