It is good to use, but needs to be filtered first. I've used it in the past and ran it through active carbon. I would also only pull from the top of the collection barrel so that all the object from the roof would settle to the bottom.
Some people use rainwater for a cheap source of very soft water. Certain fish are triggered to spawn when the water hardness drops(simulating a rain) so breeders change the water with rainwater to simulate this.