Saving for a rainy day...
You will need to store all that water somewhere. Before you pick a container, you should be aware some of the common problems rain water containers. Don't be afraid, just be informed about the choices you make on water storage.
Filtering your gutters is a great start to keeping your containers clean but you will need to clean some dirt out of your containers once a year. A container with remove lid will make clean easier. My 55 gallon containers have the standard two holes (4 inch) at the top and I have no problem cleaning them.
Another issue is mosquitoes. Some rain barrels use an 'open system' that have an open hole on the top of the rain container (with a screen to filter out the leaves and gutter items), but the mosquitoes still get in. Mosquitoes like to breed in large amounts of water, so in the 'open system' design you basically have a big mosquitoes maker. You could keep them under control using chemicals or vegetable oil or even gold fish. Close systems however filters keep the mosquitoes out, but may be more difficult to clean.
An overflow outlet is crucial to a rain barrel system. Once your rain collectors are full, the overflow outlet allows excess water to leave the rain barrel system. Hopeful you have planned away for the overflow outlet to allow the water to move far enough away from the house. In my first design, I forgot to include a overflow for my 'Trash Can' rain barrel. After a heavy rain, the trashcan lid had popped off and the excess water pooled around my house. My sump pump was running three times as much that day.
If you live in a colder climate (like me) you will want to consider how cope with freezing water during the colder months. Remember water expands when it freezes and that is bad for your water containers. You could build a heating system to fight mother nature. My solution is to remove the rain contains during the cold months. You probably won't be growing much when it get that cold. Remember to reconnect the original down spouts to get the water away from the house.
What containers do you want to use? There are some great choices at your local home improvement stores, but they cost about $200 for a 50 gallon unit. 55 gallon food drums seems to another popular choice, but require some modification to allow water intake and overflow. Most are blue and don't really look pretty. Trashcans that are made from food safe plastics are a good choice. I have even seen 250 containers, but they would be hard to move around when they need to be stored for winter. Your choice should best fit the space you have chosen for the rain barrel. Whatever you use, make sure they are clean and were only used to contain food safe products (do not use containers that held chemicals).
Remember safety first. 55 gallons of water weights about 458 lbs. You will want to make sure your heavy water container is not at risk of tipping over or falling on someone.
Keeping Mosquitoes out of your Rain Barrels:gardening tips and ideas
Food Safe Plastics: