Is an f/1.8 prime sharp enough, or do I need f/1.4?

Across the reviewers we read, modern f/1.8 primes from Canon, Sony, and Nikon are sharp wide open and often optically excellent by f/2.8. f/1.4 lenses buy you about a stop of light and slightly smoother subject separation, but at 2-4x the price and weight. Most reviewers recommend f/1.8 unless you specifically shoot low-light events or want shallower depth of field.