No, they aren't good for you. All that extra weight down there throws off your body mechanics and your legs et al have to compensate for it, all for bad effect. This info based on an old study to discover its effects. This is similar to another study about batters warming up in the batter's circle (holding and swinging) with 2 or 3 bats at a time. The idea that if you can swing 2/3 bats well, then using only 1 would be better/easier/faster. End result: batters who warmed up with 1 bat did better.
For better legs, run, cycle, leg presses, (carefully) squats etc.