We use stored rain water on our small fruit/grape farm, and more than once it's helped us late in the summer when our normal water source wasn't able to keep up with our irrigation needs. Primarily, the system is used to irrigate some fruit trees where I don't have permanent irrigation lines running to.
I have 12 55 gallon drums in my rain barrel collection system, they are scattered around a few down spouts through out or property. Each drum was free or $12-$15. I use a couple better quality submerisible pumps($120-150'ish) for transferring to a 150 gallon tank as needed that I have plumbed gravity feed to drip emitters. I could easily fill another 12 - 20 drums pretty quickly during our winters.
The pumps I use are:
http://redlionproducts.com/sump/cast-iron-sump-pumps/
Some have a float attached but others are on/off with the power. They get ALOT of use through out the year and not just in the rain barrels. I'd recommend the brand.
-Chris