Wind energy storage using pumped water in tanks

Well, the last post about weights was interesting, and shows how much weight you need to store the power of a single wind turbine. This got me thinking about the old theme of using water to store energy.
Now, what if you put a big water tank on top of a hill, and another at the bottom. How much water would you need to store 1 MWh?

Let's assume a fairly big hill, say 500 meters tall (1500 ft or so). Weight of water is 1000kg/m3. Using the same calculations as in the previous post,

Raising 1 kg 500 meters stores 10*500 = 5000J = 5000/3600 = 1.4Wh. To store 1MWh, we would need 1000000/1.4 = 720,000 kg of water = 720 cubic meters of water. From here:

http://heartlandtankservices.com/building_new_tank.html

Converting cubic meters to gallons, 750 m3 = 200,000 gallons, smaller than Heartland's smallest tank. Their largest tank holds 2 million gallons, so it would store enough power for 10 hours of wind production.
So if you have tall hills around, it might be useful to store electricity using two water tanks. I recall reading somewhere that stored water is about 80% efficient round-trip. Pretty reliable, and low cost as well, just fairly ugly, two tanks plus the pipes connecting them.