Rain barrels are common here for watering gardens. To determine if its worthwhile figure out the square footage of the collection area (roof), and multiply by the rainfall amount to see how much you can collect on a typical rain fall. Then figure out (by checking local weather stats) how often it typically rains and decide if you can empty the system between rains. Use that to determine your max reservoir size.
Typically undersized is a bit more cost effective. Larger systems tend not to fill up as often and you end up paying for empty space that might only get used twice a year vs. the first 200 gallons that gets used every time it rains.
My system is scalable, I use plastic barrels that can be connected if I want to add more on. It also cost under $0.50/gallon (100 gallons), I like it but its payoff is about 10 years, realistically there's not that much money to be saved/made because of my local rainfall patterns. Its just a good environmental project, it helps alleviate the pressure on the storm water system and saves the cost of purifying water for garden use.
