The waste water is actually a brine version of your tap water. it's full of the waste that the RO membrane pushed aside.
I wouldn't drink it, but there are lots of other uses as mentioned. Wash your car, water plants/flowers, wash clothes. I suppose if you wanted to get really serious about it, you could boil it and bathe in it
I have the luxury of living in an apartment in which the water is a set amount included in my rent.