One thing that hasn't been emphasized is how an on-demand water pump, like the one made by Metlund discussed above, differs from a circulation pump, whether operated by timer, switch, or continuous. Taco makes an on-demand pump. I like the one made by Chilipepper:
http://www.chilipepperapp.com/howit.htm
In my opinion, an on-demand system is superior to a circulation pump. It only operates as long as necessary, shutting off automatically when hot water reaches the tap. This conserves both energy and water. OTOH, a conventional circulation pump *wastes* energy. Moreover, on-demand pumps are more convenient than circulation pumps, except when left to run continuously, which is obviously a huge waste of energy.
The most common way to activate an on-demand pump is with a button. And since it operates at a much higher flow rate than a circulation pump, it only takes a few seconds for the water to reach the tap (depending on length of supply line). In any case, it's faster than trying to operate a conventional circulation pump like an on-demand system.
I live in the desert. In my town, circulation pumps on timers are now required by code. Big mistake in my opinion. Most folks I've talked to unplug them after they get their first couple of energy bills. Those that do use them typically endure the inconvenience of manually control, and even then, people tend to use them sporadically.
On-demand systems are a bit more expensive but there's a payback in terms of convenience and energy savings.
David