Last month I installed a 55 gallon rain barrel here. It is connected to one of the four downspouts on the house. I had to add 3 elbows and 3 short straight pieces of aluminum downspout but they were cheap.
This particular rain barrel included an overflow hose, so that you can easily direct overflow away from the foundation.
The most important part of the installation was installing a strainer in the gutter hole at the top of the downspout. I bought an aluminum mesh gutter strainer from a small local hardware store. Chipmunks climb up in our gutters to collect seeds and I don't want drowned chipmunks in the water barrel. (The chipmunks climb up the downspouts like a mountain climber going up a chimney formation. They make really weird metallic scratching noises while they are climbing or descending.)
Support the barrel on sturdy concrete blocks or paving stones, so that you can get a bucket under the faucet at the bottom. I added a four-foot section of garden hose so that I can fill tall buckets, as long as the water level is higher than the top of the bucket.
The first 1/2 inch of rain filled and overflowed the water barrel. I suspect that even 1/4 inch of rain would have more than filled it.
It was purchased to supply rainwater (un-chlorinated) for the garden. But normal summer rainfall here (southwest Ohio) keeps it full and there is plenty of water for other purposes.
I wouldn't drink the water without treating it, since birds do poop on the roof, but it is a handy emergency supply.
Of course the rain barrel must be disconnected, drained, and stored, before winter freezes. I kept the old section of downspout. So I can remove one screw, disconnect the new downspout to the rain barrel, reconnect the old downspout, and install the screw. The rain barrel is located to the side of the original downspout so that the original configuration can be re-installed without moving the heavy filled rain barrel, if necessary.
This rain barrel was purchased from A.M. Leonard, a local supplier of professional gardening equipment who I knew were reliable. Part number RB60 was $134.22 including shipping and tax. It surprised me that the rain barrel was drop-shipped from Oregon, and that it was made from a recycled Italian food-grade barrel. (Looks like the barrel held olive oil originally.)
The A. M. Leonard website http://www.amleo.com/ is confusing so you will probably prefer to telephone them at 1-800-543-8955 (toll-free within the USA).
This rain barrel is high-quality and I am very happy with it.
However, it seems unnecessarily complex and expensive. You can probably find or make something that is simpler and cheaper. I bought it because I couldn't figure out a good way to attach the faucet to the 55 gallon barrel that I already had. This manufacturer finessed the problem by installing a brass fitting, then screwing the faucet (actually a garden hose style plastic shutoff valve) to the fitting.