Manually operated controls schemes (or on/off systems) are the least efficient for snow removal, although they do a pretty good job of ice melting. They rely on you to say to yourself, "Gee, I heard it's going to snow; I'd better turn the snowmelt system on."
The fact that they are cold-start systems is problematic. They won't melt the snow as quickly as automated systems. If a large amount of snow has already accumulated on a cold driveway by the time you manually activate the system, only a thin layer at the bottom of the snowfall will initially get melted. The result is a dead-air space that works as an insulator. In this case, insulation is a bad thing. It will take awhile for the rest of the snow to be melted, as the dead-air space temporarily keeps the radiant heat away from the snow.
Avoid manually operated systems unless time is not an issue for you.