This is another machine that has it's place in rehab, but ends up doing more harm than good in the weight room. Much like other time wasting machines, the seated leg extension really does not factor in biomenchanics and functional movement. Leg extensions are a potentially dangerous exercise because when only the shin is in motion, the exercise draws the patella back onto the femur increasing joint compression forces, which can damage the connective tissue and the ligaments supporting the knee joint. It can also cause anterior knee pain so people with existing knee problems may aggravate them by doing this exercise.
The knee joint is a modified hinge joint, which basically means it has one degree of freedom (sagittal plane extension and flexion). The important thing to remember is that the machine will not dictate which muscles are used. Rather, muscle fiber alignment and the direction of resistance will. The quadriceps and hamstrings have a fiber alignment set up to concentrically perform extension and flexion. The entire exercise is wrong when it comes to functional movement, seeing as the quads function as reactors to gravity and ground reaction forces, not by selectively contracting against a machine.
The better option for increasing leg strength or specifically targeting the quads are squats. Hack squats, ball squats, dumbbell squats, single leg squats...there are a million different ways to do them. Plus, you'll increase your core strength and burn more calories this way than on a machine.