Auto brakes on Boeings actually are decelleration monitors. I don't have my books at home to give you the percentage of decelleration for each setting, however, they appply wheel brakes in relation to the selected decelleration rate.
In a nutshell they add brakes to normal drag and reversers to give you a preselected rate of slowing. I like to use them because it is the best way to insure you are goin to meet landing perfomance data for a given runway. Most performance calcultions for required braking use the autobrake decelleration rates to creat Min Med and Max as reccommendations for braking.
Lufthansa did a study a few years back with Boeing on AB vs manual braking and found that AB were much better at increasing brake life. Not a cost factor though, most airlines pay per cycle for the brakes under contract. They are much cheaper than vacuuming the runway with the CFM's and damaging the blades at low speeds. The study was published in Airliner mag by Boeing in the early 90's.