It hits the muscles you're focused on preciously. The amount/type of resistance is easily varied with terrain, speed and gear choice. Perhaps most important, it's sinfully fun.
Cycling, from casual rides to micro-races of 15-20 miles with friends, did more for my quads than all my previous activities combined.
I don't know your cycling conditions, but you don't have to do the Tour de France for great legs , a stationary bike will do just fine. Make sure the seat hight and crank length are a good fit for you and have fun.