I have never been a fan of tank water, there is a lot of crap growing in water tanks.
I used to live in a house that had the water tank (green poly) plumbed into the laundry and kitchen on a pressure pump. The owner told me "it runs a bit green for the first few minutes but after that its OK to drink" A bit green!!! It was greener then the grass for 5 minutes or more, no way was I going to drink that shit. I used it to wash the clothes with until it stained the washing machine intake.
Without serious filtration, 1 micron then carbon black, and possibly some UV or ozone treatment, I would be very wary of roof run-off.
In saying that I agree that mains water is not perfect either, I work in a water testing lab, and we get samples from the public cos they are concerned, this resulted in a towns water supply being turned off due to cyano-bacter in the reservoir. A local brew-by-U has been complaining about the particulates in their supply and said he is changing his filters more frequently.
I have been thinking about getting a 2 stage filtration system for my drinking/brewing water, maybe a 1 micron and then a carbon cartridge,
like this one on eBay, what are the thoughts out there?
BTW, the guidelines for mains water are much tighter then bottled water, and my boss is of the opinion that bottled water is a total con.