Watch out for Acid rain, that shit will have your plants tripping for days (Suffer a ph imbalance possibly).If you live in a big city collecting rain water might not be your safest bet, but if it works out for you then great
I got one of those filters that attaches to the kitchen faucet and use it. They filter out more than brita and way easier to use since you can stick a jug under it and let it fill. I got it for my homebrew but now have a wonderful second purpose!