My husband set up our simple rain water collection system over the weekend working through the on and off showers we had. We have already collected a fair amount of water. We use this water for our potted plants and our garden. Obviously it is not enough water to take care of the garden completely, but it does reduce the amount of tap water we use. We may in the future add another collection system to the house or garage, using a more attractive barrel.
My parents have a more sophisticated collection system. They live in the country and have a large shop on their property so they are able to collect a lot of water, just from this weekends rain (less than 1/2 inch for them) they have half-way filled their tanks. My father connected the two tanks so they fill add the same time and he hooked up a pump and pipe running to the garden so they can connect a sprinkler at the garden. They should be able to water their garden using only rain water if we have normal amounts of rainfall. Less than one-half of their shop is collecting water, so they have the potential to collect more rain if they need.