I think having any kind of rain barrel is a benefit, but for those that 'em usually want to push it further and further (especially those with a large garden). A 55 gallon barrel doesn't hold that much water (some say will fill in less than 5 minutes). For a much more elaborate plan, check out these guys. They have everything you need (for a price, of course). 30, 55, 75, 80, and (*gasp) 214-gallon rain barrels.