Do I need an expensive f/1.4 prime or is f/1.8 good enough?

Reviewers consistently say f/1.8 primes deliver most of the practical benefit at a fraction of the cost. Faster f/1.4 lenses like the Sony FE 35mm f/1.4 GM win on wide-open rendering, build and bokeh, but several community threads note diminishing returns once you stop down, so the upgrade matters most for low light and professional work.