RK2 can control just about anything you want on it's power strip (8 plugs max up to 15 amps total). They do control lights (or anything else with a plug) via timing programs you use on the unit, but they also monitor/control pH & Temperature, so you can turn your lights off if the temperature exceeds a certain level, turn fans on should a certain temperature be reached, amongst other things.
It is an expensive light timer if that's all you use it for... hell I bought an even more expensive light timer But if you start monitoring and controlling pH & Temp it's no longer that expensive (just check out various temperature & pH controllers).
So it's expense really depends upon what you're going to use it for.