I've heard both good and bad things about using rain water to water your gardens and flowers. Do you collect rain and use it? My parents did this and I remember the number of dead bugs on the top of the barrel.
We have a couple and my wife waters the flowers with the water. Standing water will attract bugs. I don't use it to water our garden but that's just because we don't get enough water in the barrels.
We use these for watering all the flower beds too. We don't water the garden with it, but I would if there was enough water. I don't think there is a problem using the water. Our flowers thrive so I'm sure the water isn't hurting them.