I'm sure by now most people know the environmental advantages of having a rain barrel. Not only is it good for the environment, it's great for your plants too! Rain water is much better for plants than municipal water, especially potted plants. Municipal water contains chemicals that can build up in the soil and harm sensitive potted plants.
I mainly use my rain water for watering my potted plants and filling up my ponds when they get low; but it could be used to water the gardens as well. Rain water could also be used to fill up wash buckets for tasks like washing the car, washing windows, or other household tasks (which saves on your water bill!).
The first thing to think about when you get a rain barrel is where to put it. You want to make sure it's easily accessible, on fairly level ground, and elevated.
The elevation will make it easier to fill watering cans and allow gravity to help out with water pressure. I use cinder blocks to elevate and level mine, and a cut off piece of old hose (recycle old junk!) to make it easier to fill my watering cans.
You will also want to make sure to put the barrel in a place where overflow won't matter. It's amazing how fast they fill up with very little rainfall. Most rain barrels have an overflow valve where the excess rain water will drain out when the barrel is full, but when there is a heavy rainfall, excess water can bubble out the top of the barrel rather than out the release valve.
If you have concerns about water leaking into your basement, you can buy an attachment that will allow the excess water to flow through your gutter like it normally would, once the rain barrel is full.
Another concern with rain barrels is bugs. Here in Minnesota we have to worry about mosquitoes, and standing water is their favorite breeding ground. But no matter where you live, you will want to protect the water in your rain barrel to keep the bugs and debris out.
My rain barrels are the kind that fill up through the top, which is wide open. To protect the water, I found some old screen material in the garage and cut it to fit over the opening, then secured it under the lid. It's as easy as that.
To overwinter my rain barrels, I completely drain them and remove the spigots before freezing temperatures arrive. Since I don't have room in my garage, I store them in a protected spot behind my hedges; upside down with a brick on top (err, bottom) to hold them in place for the winter.
Rain barrels are very popular and easy to find where I live. They are widely available at farm and garden stores or on the internet these days too. Some cities will even sell them at a discount to encourage people to go green and preserve water. Some people even make their own rain barrel.
In my opinion, having a rain barrel is a no brainer. I only wish I could use mine year round.