Regarding drinking it… Treat it as grey water. Aside from excrement on the roof, you have no control over the quality once it’s collected in the barrel. Especially in summer times, bacteria, algae and other micro-organisms will thrive in stagnant water. If there’s animal excrement in there, it could turn into a potential source of physical suffering… I wouldn’t drink it without at least filtering and boiling it. (Yay, botulism! Okay, that’s maybe not the real threat, but there’s some potential for stomach aches in there.)
That being said… It’s great for watering your garden! For toilet use, you wouldn’t have a lot of issues with high turn-over usage or maybe some sort of water purifier or granular filtering (a slow sand filter might be a good one), as there’s always the risk of algae build-up, which could clog piping to your bathroom water storage.