The amount of mental overhead that type hints reduce is staggering. I love being able to interrogate a variable, function or class and learn about its properties right in the IDE, or have autocomplete let me mangle the name of an attr and tab-complete and correct it (regardless of lang). The alternative is going back and forth from documentation to code, and having to type everything perfectly.

Even just a smattering of type hints helps dramatically, as a few well placed hints on the APIs I know the least, gives me a huge boost.