Well, I have heard good and bad - and it really comes down to how often they are serviced, and how hard the water is that is going in - best bet would be to get a gallon and test it using a TDS
meter so you can see how effective the RO
membrane is on the particular machine. If it is only a few ppm TDS
, it would be fine to use. If it is much higher, than it won't provide much benefit over using tap water.