I did a bunch of research on this a while back...sorry I don't have any of the relevant links to share with you as I lost track of them.
The bottom line I came up with is that unless your roof is metal, don't use the water run-off unless you also filter the water to remove bacteria, parasites (from wild birds) and chemicals (leaching from asphalt or wooden shingles). Arsenic is commonly used in wooden shingles. I don't recall the verdict on ceramic tiled roofs, but I imagine they would be similar to metal.
If you roof is metal and you don't have a lot of wild bird droppings on the roof, then you would probably be fairly safe using the water straight. But it would be a good idea to also drain out and bleach the barrel perhaps once a month or two.