You're going to want to purify the crap out of it if you're planning to drink it - REGARDLESS of roof type.. so go for it.
If you plan to just use it to watering the garden and stuff like that.. no need to worry about it at all. no diff than regular runoff in your yard.
Chlorinator / dechlorinator is all that is needed for drinking as long as you dont suck the chunks up off the bottom.
I would not be surprised if the bacteria and lack of getting rid of it easily is what caused cisterns to fall out of favor way back.
Or lack or rainfall!
Depending on your garden size and rainfall during the growing season, you would probably need a large large container, doubtful that 55gal drums would be worthwhile in your climate.
If your garden is 20x40' for example and you want to put down 1" of water, you'll need almost 500 gallons!