Being in drought-prone Texas, we harvest rainwater for our gardens, and to water our hens. We have three 55 gal. barrels, two with downspouts off the house roof, and one off the workshop. We also have three 250 gal. fiberglass cubes that collect rainwater off the barn and off the garage. The barrels are dip and carry, the cubes have a spigot for filling buckets. We find that there is less waste of water if we water plants individually, using either large watering cans (2 gal.) or 5 gal. buckets.
Our method isn't really economical, but we have water for our gardens when there are water restrictions due to drought, and rain water is better here than ground water. Within a few years we think that our water collection system will be economically viable.