For those interested, this is our rain barrel system. We took video to explain it, haven't gotten around to editing it yet.
We have 7-55 gallon barrels all hooked together. They fill at the same time and drain together. We have one spigot to fill watering cans and a pump with another spigot for use with the hose or sprinkler.
There's an overflow as well, they're all hooked together and overflow into the downspout if they're full. We have a clear tube at the end so we can see how much water we have. The system is also easily expandable if we want to add more barrels, we think we can fit 7-10 more across the back of the garage.
One of the best things about this system is the cost. We only spent about $120 for the entire system (including deck). The deck was built with lumber we removed from the deck on our house, we wanted a smaller sized deck on the house. The barrels are food grade plastic barrels and cost us only $10 each (you want to make sure you don't get barrels that had chemicals in them). We spent about $50 on other materials, posts, PVC (we also had some of that sitting around that we used up).