Yes in general it can be quite good tends to be soft, I imagine it is not at its best when it first starts falling and you may not want any that is immediately downwind from that large foreboding chemical company/superfund site up the block. Is there something wrong with your tap water?
I allow a couple of storms to wash the roof, then collect and use the rain water in season. It does start to soften the water in the tanks, as it dilutes the minerals. Any tanks that should not be softened (Lake Tanganyikan and brackish water tanks) get some minerals and salt added that keeps them where they ought to be.
Careful also if the roof is toxic. For example, a rain collection system that has a lot of copper is not good for shrimp and snails. (I remember a post from someone with copper sheeting over the roof.)
Take care when collecting the water that it does not stand too long and start to grow mosquitoes. Feed the mosquito larvae to the fish, of course, but better to use the collected water quickly or cover it so you are not breeding the little pests.
These days, if you live anywhere near a city I wouldn't use rainwater. There are normally so many dissolved chemicals in the water it's not funny. You're better off getting a cheap RO unit if you want super soft water.
Probably need to add some calcium source. You can use the same stuff used for RO water re-constitution.
But I'm with Phil. Unless you live out in the country and the rain actually rinses things clean (no spots on your car after a rain) I'd avoid it because of the chemicals in the atmosphere that mix with rain water in the city areas.
I live just ten miles east from st. Paul Minnesota and hence just a bit farther from Minneapolis and have had no problems using rain water. There are some rural areas and suburban areas I would be more concerned about in terms of polluted rain water. My big concern is the manufacturing/industrial corridors that often surround or are close to a city. Rain also washes a lot of dust from the air and that is often the source of deposits on cars. There are erios and isoetes growing all over the shallow non-spring fed lakes around here and they do not seem to be suffering from the rain water. It also helps to let the rain water sit in a barrel for a while to allow any precipitate to fall out of it. It is also good to not collect the water until after it has been raining for a bit. Though precipitation may not be reliable in some parts of the country, it is pretty consistent up here.
I add minerals for the hard water tanks, but not the soft water tanks. Rain is seasonal, so my soft water tanks go through an annual cycle, getting somewhat softer in the winter and harder in the summer. These tanks do not really get as soft as I would like, though. Rain is too unpredictable here, and I still do tap water water changes in the winter, too.
I use Seachem Equilibrium and baking soda for my hard water tanks.
Systems that collect rain water for watering lawns and etc, have by pass systmes to where the 1st X ammount goes down the drain. you would be amazed the ammount of dirt that collects on on a roof and in your gutters.
I have used rainwater now for about 18 years for my planted aquariums.
I collect from the colour bond roof into large plastic tanks, which we use for the house water as there is no mains water. I added a smaller tank solely for the aquarium room. What I do is filter it first through a 3 stage filter bought from local hardware supplies into a smaller tank just to remove any bulk debris or nasties.
In this small tank I run large air stones 24/7. Aerating the water dissolves Co2 and raises P.H. As the Co2 content is high naturally it is no good for my shrimp.
I have large systems with sumps so I just top up the water as required. In individual tanks I add GH and KH as desired.
Now I am moving to an area with mains, so I will probably stop. But over the years I have never had any problems. In fact, in my plant tanks I never aerated the water until I started with shrimp. They would not breed well in rainwater.