This Old House did a segment on using them a while back. Here's a link to one of their articles: http://www.thisoldhouse.com/toh/article ... 89,00.html
We are fortunate to live on an old farm where we have 2 underground cisterns (sp?) Each is fed by a rain gutter. One off the barn, the other off the entire south side of the big ole farmhouse. These babies are about 10' in diameter and 30 ' deep. They had been getting low but the past 4 days with over 6" of rain has filled them back to the top.
We installed shallow well pumps to each that have been supplying water to the flower beds and garden, and even keeping the pool level where it needs to be.
Any type of rain collecting devise is to be considered eco friendly and well worth the investment.