I hear your discussion regarding the collection of rain water and would like to add that it is important to use a system that does not facilitate an explosion of mosquitoes.
Mosquitoes are vectors which carry a number of diseases and it is important to control the population.
Pls check out the Facebook page for Marin/Sonoma Mosquito & Vector Control District for examples of good and bad methods for collecting rainwater. The good methods make it difficult for mosquitoes to breed.
Links:
[1] http://www.moneypit.com/blog/leslie-segrete/rainwater-collection-saves-money