Nvidia claims Reflex reduces the time between pulling the trigger in Overwatch and the game rendering the shot on screen by as much as 50 percent, and now you can easily test that.
Your mileage may vary, but if you have a Nvidia graphics card — Reflex works on cards as old as the GTX 900-series — and a G-Sync monitor, you should be able to give the low latency feature a try.