There are lots of commercialy available water capture barrels that sit under a downspout from guttering. The purpose is for watering plants and conserving water. Also, in many third world high rainfall areas, many or most of the houses are equiped with a rainfall capture system (cisterns) and that water is used for all household and garden purposes. So anyway, rainwater in general is better than most other sources, no chlorine, no hard minerals, no salts. Great idea, use that rainwater!
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.