Those who own rain barrels for water collection have probably never thought to drink the rainwater. It probably goes on the garden or into the power washer. Drinking rainwater isn't that great of an idea, especially with acidic rain and the like. Plus, I'd be hard-pressed to drink anything that rolled down my roof and into my gutters.
There is a way to turn filthy rainwater into sparkling drinking water using sand. To me this is ironic because I consider sand to be dirt's equal. Using what is essentially dirt to clean water seems to be counterintuitive, but the process works according to the Carbon-Free Home. Chelsea Green recently posted on this subject. Check out their diagram.
Basically, water enters the barrel. In the middle of the barrel is a layer of sand. The water above the sand is biologically active. (Don't drink that.) The microbes in that layer eat many of the impurities and biological contaminates in the rainwater. Several feet of sand do the rest. The water slowly trickles through the sand and at the bottom you have fully drinkable water. The top layer of microbe-infested water will have to be removed from time to time, but otherwise, you've got yourself a low-maintenance rainwater filter.