Is it useful to control speed by gearshift instead of brakes?
It is use full in the right situation.
If you're going down a long steep hill you should leave your car in 4th or 5th and let it cruise down the hill at its own speed.
This saves the brakes and also lets you have a more controlled descent, if you have a large load in say for example a truck you should definitely use your gears as your breaks could overheat.
It will increase your overall fuel consumption a little if you use engine breaking (that's what slowing down with your gears is called) often, but depending on how expensive your breaks are it could be more economical.