This very day
our builders have connected water butts to all our new downpipes, and they've also connected a butt to the pipe that carries the water from our bath/shower/washbasin. It's fine for watering direct onto the soil of veg and flowers. Next year I plan to attach a hosepipe to the outlet and connnect a seeper-type hose to that and leave it laying between the rows of runner beans.
I wouldn't water the leaves of plants that you're going to eat as it may leave a residue but as I said, watering the soil direct is fine. It's certainly fine for ornamental and shrubs etc, even with soapy stuff in it. We tend to use very mild shower stuff and shampoo but I don't suppose that any stuff you're prepared to put on your skin would damage the plants if it's well diluted.