The function of Thrust Reversers on the CFM equipped 737 is to make lots of noise and consume fuel.
Actually they are pretty effective. This (incorrect) attitude comes about because the autobrake schedules a set deceleration rate - so when you hammer the reverse, the autobrake reduces the braking effort. The pilot feels the same deceleration with the reversers roaring and assumes that the reversers aren't doing anything.
Next time you are on a decently long runway, land with the autobrake off, your feet OFF the brakes, and use full reverse to see how quickly you pull up.